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  • Andrew Humphreys Level 1 Level 1 (45 points)

    After a week of testing with no graphics glitches or crashes whatsoever, I think it's safe to say I've fixed the problem on my machine. The "fix" is basically a do-it-yourself firmware upgrade for the graphics card, in my case the ATI Radeon HD 5750 (really a Mobility HD 5850) inside my mid-2010 iMac. It's a fairly straightforward fix, although it does require you to use a Bootcamp partition to run Windows on your machine and it's not for the faint of heart. Flashing the BIOS (firmware) of a card (or anything else) can be risky. If something goes wrong during the process you might not be able to boot your machine. Chances are everything will be fine, and for me it was a very smooth process, but you should always be prepared for the worst.

    Be aware, too, that this is a fix for a very specific problem. Ever since the Lion upgrade, my iMac has been experiencing graphical glitches, especially in Finder windows (Quicklook previews were a disaster), although they also started spreading to Mail windows and Safari menus. I kept hoping a Lion update would fix it; instead, it kept getting worse. With 10.7.4, I also began to experience the crashes that others have described in this thread, where the screen would freeze (usually while showing odd square-shaped artefacts) and only the mouse pointer would move. A hard re-boot was the only fix. Console logs showed this was a problem with the GPU (GPU Debug Info messages appeared every time).

    I'd always assumed the problem was Lion specific, but a post in this thread showed me the crashes could occur in Snow Leopard, too. So I started digging around and it turns out the exact problem also appeared for users running the same ATI card in Windows machines. Lion may have revealed the problem, just as Windows 7 apparently revealed the problem for PC users, but it's really a problem with the GPU. In a nutshell, the problem is this: the BIOS of certain ATI HD 4XXX and 5XXX cards sets the power-saving idle clock speeds too low. Whenever the GPU is required to switch to a higher speed (for example, by drawing a Quicklook image preview), glitches occur and stability is compromised. I think Lion (and Windows 7) allows the GPU to enter the idle state way more often than Snow Leopard did, hence the increased frequency of the problem.

    Check these links for a discussion of the problem:



    On the PC side, ATI driver updates and in some cases BIOS (firmware) updates have fixed the problem. You can also use ATI's Catalyst Control Centre on a PC to tweak a card's core and memory clock speeds without having to flash the BIOS.

    I'm not sure that a driver update from Apple will fix the problem. There have been plenty of updates to the ATI kext files since Lion's release and the problem remains. It may be that the drivers can't override the speeds set in the BIOS, in which case a firmware update for affected cards is needed. But Apple doesn't appear to be in any hurry to release a graphics firmware update for mid-2010 iMacs, which is why I decided to do it myself. I was having so many problems that my machine was basically unusable, and I figured I'd need a new graphics card anyway if the fix didn't work, so what did I really have to lose?

    As luck would have it, the tools to extract, edit and re-flash an ATI graphics card's BIOS are readily available to Windows users. There's also an excellent guide to the entire process, which I read and followed step by step. The guide is here:



    Essentially, I booted into Windows 7 (that's why you need a Bootcamp partition - virtualisation software won't give you direct access to the hardware) and followed the guide.

    Here's what I did:

    First, I extracted the BIOS using GPU-Z (available here:

    Then, I loaded the BIOS file into Radeon BIOS Editor (RBE), available here:

    Radeon BIOS Editor gives you all sorts of interesting information about your GPU's BIOS, but the most important tab for our purposes is Clock Settings. The Clock Settings tab shows the clock speeds and voltages of the various PowerPlay settings (the boot mode and various power-saving modes, for example) on the card. The GPU speed (in MHz) is the core clock speed; the RAM speed is the memory clock speed.

    Sure enough, the lowest power-saving setting of my card was set to 157 (GPU) and 295 (RAM) - the same setting that had caused problems for Windows users. So I changed it to 398 (GPU) and 1000 (RAM), which matched the next lowest PowerPlay setting in the card and was also close to the suggested fix from PC forums (from 157/300 to 400/900).

    I then saved the modified BIOS and flashed it the card using ATI WinFlash (available here:, rebooted back into Windows to confirm the changes had taken, then rebooted back into OS X.

    I've been running glitch-free ever since - no graphics artefacts, no crashes, no problems of any kind. It's only been a week but I'm certain it's a permanent fix. Before I flashed the GPU I'd had three consecutive overnight lockup and I'd been having problems with glitches and graphics artefacts every day. Every time I clicked on a Finder window I'd regret it; every time I opened an email with an embedded image I'd flinch. If the problems I'd been experiencing for months on end were going to resurface, I'm sure they would have resurfaced by now.

    I've been running Hardware Monitor to keep an eye on the temperature of the graphics card heatsink and diode. Obviously the idle speed of the GPU is slightly higher now, but the temperatures are well within the normal range so I don't expect any problems to arise. Right now, everything "just works".

    As I said, this is not a fix for the faint of heart. If you're not comfortable with tinkering with your hardware, don't do it. I've made Apple engineers aware of it and hope they'll release their own firmware update for affected cards.

  • fvd Level 1 Level 1 (5 points)

    @Andrew Humphries Thanks for a very thorough and extensive post. Hopefully it will make some readers of this thread think and profit them. I have to say that it is far too technical for me and many, BUT that is probably reflective of what we need now--something that goes deeper than just scratching the surface regarding this problem. Unfortunately, I am in the pool of many here: I will have to wait for Apple engineers to release a firmware update, as yo say, because I just don't have the technical savvy to proceed along a route as you did. Hopefully you will come back to this thread now and again to offer your expertise even though the problem seems solved for yourself--even though you made your finds to Apple. I have the fear that again Apple will leave us on our own and marginalize us and the problem.


    By the way, thanks for noting that the problem is not exclusively that of those who have Lion and a 2011 27" mac. I do not fall into that 'Lion + 2011 +27 inch mac"category, yet I have insisted on being a part of this thread. I have Snow Leopard 10.6.8 on a late intel 2007 mac and experience what has been in this thread. As you note in your response, I also  have freezes/crashes most often when coming out of sleep, and have long suspected that there might be a problem related to power saving involving sleep mode. I refuse to go on to Lion until this all is cleared up, and your response seems to support that in that you say that things got progressively worse when you updated and went further into Lion with upgrades.


    Thanks for all your dedicated work, and I hope that someone at Apple takes note of it and goes further with it to solve for us the issues involved.

  • Andrew Humphreys Level 1 Level 1 (45 points)

    @fvd. Believe me, I'm no technical expert. But the graphical glitches had been driving me mad for months and with each update from Apple things didn't get better, they got worse. I tried everything I could, including replacing the Lion drivers for my graphics card with older versions, but nothing worked. Eventually it clicked to me that my problem was a GPU problem and I stumbled onto a working fix.

    As I said, the fix I described above is for a specific problem and specific ATI graphics cards. It won't work for everybody.

    It's also clear to me that there are wider graphical anomalies across a broad range of machines. And I keep reading forum posts from users beta testing Mountain Lion saying that the problems still persist. Apple needs to get its GPU firmware and graphics drivers right, and fast. It's incredibly frustrating to have machine that refuses to work as it's supposed to.

  • rubeandg Level 1 Level 1 (0 points)

    This problem has also been driving me crazy for the past few weeks, to the point where the screen was blacking out after just 3 seconds. The Ctrl, Shift, Eject, random key combination worked but every three seconds was a bit much to bear. I remembered that I once reset the Ram and it helped keep the system stable so I did this again and (touch wood) not a single problem in the past 24 hours.


    Link here:



    Hope this helps.

  • jjjjjjb Level 1 Level 1 (5 points)

    I think you might be correct @AndrewHumphries in your pinning it down to the BIOS of the graphics card.


    Last night the screen in my 2010 iMac decided to shut itself down when I set the computer to a CPU-intensive task.  I turned on the secondary screen and it was full of graphics corruption.  Soon it shut itself down too. Impossible to get it back.


    I have noticed that this sort of corruption only occurs after the computer has gone to sleep and been awoken - i.e. since I've restarted it this morning everything is fine.  Which would confirm your points re. the idle-time settings of the graphics card.  I am still on Snow Leopard so it's not a Lion-specific problem, but rather a hardware problem.


    So it would seem that flashing the BIOS as you suggest would be the way to go.  (Of course, for some reason, Bootcamp won't work on my iMac... so I hope Apple fix it!)

  • Andrew Humphreys Level 1 Level 1 (45 points)

    jjjjjjb, I think our iMacs are exactly the same - same GPU, etc - so it's a shame you can't get Bootcamp working. Be nice if someone else could verify the fix. Coming up to two weeks now and I've been running the whole time without a single issue. And I still haven't heard anything from Apple.

  • luibargi Level 1 Level 1 (0 points)

    I collected today my iMac from an Apple service shop.

    It is Ok now but the LCD display had to be changed as found faulty.

    There was no firmware or other OS related problem, it was an HW failure.

    So my LCD worked for 16 months only, then broke down.

    Unfortunately no guarantee was given as I bought the iMac as a company (1 year guarantee) and not as a private user (2 years guerantee). Total bill: 495,00 euro plus VAT 21%.

    I am very disappointed even though it is the first time I had problems with a Mac after I switched from windows four years ago.

    Clearly the display was defective but I had to pay for the new one, owing a 'bureaucratic' reason.

  • TWKP Level 1 Level 1 (0 points)

    The Apple store diagnosed my iMac and determined that the hard disk had failed. The drive was replaced and I have been without incident now for 12 days.


    I have been following the recent posts and I don't think that I have experienced the same problem regarding the video GPU timers (or maybe there are multipule symtoms for the same root cause). My problem first surfaced as a problem where I would loose bluetooth connectivity (on Snow Leopard). This was most noticable when I was on a skype call with a bluetooth headset. Not only would I drop the call but my wireless mouse and keyboard would disconnect from the iMac. My work around was to access my iMac desktop via my Mac Mini and after several attempts to reconnect the keyboard and mouse (in most cases) I would be back in control. In some cases I had to close apps and reboot the iMac. In rare cases, the iMac would be frozen and I would have to power cycle the unit to get it back.


    This "freezing" of the OS progressivly got worse and it was very noticible with Lion. The problem persisted for close to a year and as it started out as a minor problem occuring only once a week or so, I did not take it too seriosly at first. It progressed to the point that the machine was unusable and just prior to taking it to the Apple store I noticed that my Hard Disk had failed the S.M.A.R.T. self test.


    I have never allowed my iMac to go to sleep except recently as a test based on the feedback in this forum. So the changing of GPU clock speeds should never have been a factor for me as far as I know.


    I have installed the smcFanControl to bring the temp down a little.


    For now the machine is working.

  • iDik Level 1 Level 1 (0 points)

    iMac 27" late 2009 10.7.4 had black screen with 2nd screen working and later on full blackout all screens with sound carrying on playing.


    Machine runs 24/7 for more than a year

    Display sleep set 15 mins

    If I'm away from computer I do manual sleep.



    After java disable and smcfancontrol I've had no crashes for months

    the machine has been running currently doing things for 7 hours and rear of casing seems relatavly cool.


    I can trigger the problem if I wish by booting into windows and loading Fallout 3. I then have a couple of minutes before it goes off and sound remains.


    Out of warranty.

    I will ultimatly replace the graphics card myself if a decent affordable upgrade appears.

    If not hopefully a firmware patch will come from Apple or the graphics card vendor for the graphics card


    For now, as I dont push the gpu, I have had stability

  • brandtdary Level 1 Level 1 (0 points)

    I think this may be the same thing I'm experiencing... I am curious to know if any of you that are following this will clarify if this is the same bug I'm experiencing.  How will you know?  When ever this type of thing happens to my 27" Core i5 iMac, I am able to unfreeze it by pressing Volume Up, Volume Down, Mute, Chang the screen brighness... anything that brings up an "overlay" dialog from Mac OS (as seen here).  Does this 'work around' work for anyone else here?


  • fvd Level 1 Level 1 (5 points)

    @brandtdary: the freeze just happened to me and your workaround did not work for me. Except for the cursor, things were completely frozen regardless of what I do. Only opption: a hard turn off.

  • TWKP Level 1 Level 1 (0 points)

    I think it's safe to say that I'm back up and running after having the HD replaced. My 27-inch, Late 2009 iMac is stable and I am using it on a daily basis for my business.


    I had several symptoms over a long period (over 1 year) of time and I think that this thread had some of the same symptoms but in the end the problem I was concerned with was not related. I do believe that I have hit several bugs along the way and in the end my HD failed. This was a real rough patch for me and it affected my ability to work on my business for an extended period of time.


    Having said that, I have discovered that I do suffer from the GPU timing issue as described by Andrew Humphreys. As I have (almost) never used sleep mode, I never realized I had the problem. With the info from this thread I can re-create the problem on demand. Thanks for the bootcamp tip but my preference would be for Apple to provide a fix as I don't have bootcamp installed. Maybe I'll get around to the bootcamp fix in my spare time.


    I also installed smcFanControl just in case temp was an issue and the iMac is running at 35 degrees C.

  • chkar Level 1 Level 1 (0 points)

    I have the exact same imac modell and have been suffering by the same problems everyone is mentioning in this forum. After many unsuccessful searches I ended up to your post, which was very enlightening. I have just followed your instructions and managed to flash the bios of the imac's ATI graphics card. Everything now seems to be fine. I will let you know if any problems reappear or if they have left me on a permanent basis (which I sincerely hope!).

    Many sincere thanks for an excellent and very clear post, as well as for sharing with us the great amount of personal work you invested on this!

  • Andrew Humphreys Level 1 Level 1 (45 points)

    Glad to hear the process went well. And I'm confident it'll be a permanent fix for you. It's been over a month for me now with no graphics glitches or crashes whatsoever. I'm really enjoying using my machine again.

    The procedure I outlined is really very simple, especially if you already have a Bootcamp partition on your machine. All it takes is a couple of minutes and a willingness to tinker:-)

  • Geoffrey Hooke Level 1 Level 1 (5 points)

    iMac Pro 2009: 3 months of the deadly spinning wheel of death forcing multiple restarts every day. Tried everything suggested here to little effect. Yesterday I deleted 2 anti-virus programs - Eureka! Haven't experienced a spinning wheel in 24 hours - first time in 3 months!!