Imac display issues following Yosemite upgrade

Oh I am going to test it (Diablo III) alright 😉

Pixel anomalies like that are not normal, should definitely take it in and see if it is covered under any of Apple's repair programs.

I had the same issues and I tried to isolate the problem by determining what was causing it. Now, with Mavericks just a few days ago I had installed an external RAID and with Mavericks it would work perfectly well, so, I thought it might be the drive and I tried unplugging it along with all other peripherals. That did not solve it. I noticed that the issue of pixelation and eventual freezing would come up primarily during video playback. I am using VLC, so, I tried using quicktime or Divx video players and that did not solve the issue either as it started coming in during youtube videos. A couple of friends suggested PRAM and SMPS reset and I did those things too and the machine became stable for a few hours and then the issue came back. I also tried to switch browsers from Chrome (to firefox) and check if that is causing some issues, but, no. So, finally I'm switching back to Mavericks until this issue is resolved. The methods of reverting back to Mavericks are not easy but I have no choice. I suggest you do the same

Just to continue, experienced the same issue after updating to Yosemite on Imac 27" i5, Mid2011. I had Fusion 4 and EyeTV Lite installed, that would't work after update. Removing the applications didn't resolve the issue. Unfortunately I have to do some work on my Mac, so I'm going to restore previous OS. If no solution comes around in time, I'll remove these applications before running the update to Yosemite and see, what happens.

There's a pretty long feed about this issue here, mainly 2010 iMacs with the ATI Radeon 5750 chip:

https://discussions.apple.com/thread/3867557?start=75&tstart=0

I think this is a new issue more related to software. I have had no problems with either Lion, Mountain Lion or Mavericks. My display issues started as soon I upgraded to Yosemite and stopped as soon I went back to Mavericks.

I had considerable success eliminating this problem on my mid-2010, iMac i5 27" simply by turning off all energy saving parameters: Worked great through Mavericks, and all the former green/purple pixilations ceased.

Then I installed the upgrade to Yosemite.... and they returned, with a vengeance.

For the first time, however, I received the alert stating "video card problem", which then proceeded to send my anonymnous report to Apple, so I - too - am cautiously optimistic that Apple has indeed recognized and is investigating this long-running issue that SO many continue to experience.

For the first time - following the alert message - my iMac actually froze completely, forcing a hard restart... which itself had to be "restarted", when the process stalled.

Once my system returned, it was like watching the initial scenes from The Wizard of Oz - where Dorothy awoke across the rainbow: I have never seen such rich, vivid colors - nor such depth and crystal clear, sharp imagery as I'm experiencing now: NO PIXILATIONS WHATSOEVER, and a gorgeous display!

I do, however, believe my iMac's running more slowly under Yosemite...

Hopefully, I'll be able to stave of baking my card: Replacement prices (rare, and harder to find these days) seem to be all over the map, and rising...

Thanks for everyone's technical expertise input on these two forums: You guys rock!!!

I'ld like to confirm the statement of enricoclaudio, restore OS to 10.9.5 - no display glitches any more.

Did a new update to Yosemite after removing all applications, which seemed to be not compatible with Yosemite (VMWare Fusion4 and EyeTV Lite). Square flickering spots are back. After changing the Monitor profile to "Generic RGB Profile" it became much better, but still the spots are appearing from time to time. My frustration level is not reached yet, so I'm going forward with Yosemite. Hopefully the Apple people come around with a solution before I loose my nerves and throw the box out of the window. ...and no, I will not bake my video card.

I did my last attend yesterday. I did a 300GB partition in my internal 1TB HD, did a clean install of Yosemite which BTW took only 16 min. My other installations lasted between 45 - 55 min. I didn't install any other application rather than Yosemite itself, not even Firefox or my Epson printer. After 45 min the display issues started. I have Mavericks in my 700GB partition running perfect, so this is definitely a driver/software related issue.

That's weird because I had the same problem with my Mid 2010 21'5 iMac running Mavericks, but now with Yosemite the display is crystal clear. Is still get this error message in my log though:

kernel[0]: BUG in process suhelperd[313]: over-released legacy external boost assertions (1 total, 1 external, 0 legacy-external)

And today it froze for the first time. I didn't have my phone close or I would have gotten a picture- the pixellating was over the whole screen. Never seen it like that before. Had to do a hard boot and when it came up - I had this message:

Arghhhh!

I got that same message three days ago and because of that I decided to stay with Mavericks.

I've been having this issue for a year, and started with Mavericks I believe. It's so bad currently that my mid-2011 imac freezes almost daily, forcing me to turn it off and on to get it back running. This issue was certainly present ahead of Yosemite. For me, it wasn't related at all.

Took it to the Apple Store a few months ago, and of course it started working perfectly in store. They ran diagnostics on it and everything came up OK.

The imac has seriously been a lemon from day one. It runs so slow, and the graphics issue just makes it worse.

Now I get a graphics problem was detected message on a hard restart.

Could this issue be related to memory upgrade? I have mine running 2 slots with original 2GB and 2 slots with Corsair 4GB for a 12GB total. Maybe my Corsair memory is not compatible?