27" imac flashes colored pixels and freezes

I have seen this trouble today in 2017, I know it's a bit late but many users could experience this these days. I have found a solution. It's a software problem.. I have tried everything and nothing worked, the only solution is this:

-Plug an external HDD to you computer and backup all data manually ( no time capsule )

-You download from the app store in the purchased section OSX Maverick (the one I had)

-With Diskmaker X create a bootable installator of maverick

-Turn off computer when it's done

-Turn on computer with cmd+shift+r pressed on

-Go to disk utility and select your internal HDD and erase ALL the partition to have a brand new hdd

-When it's done return to the menu and select install OSX( with the usb key you create)

-Follow the instruction and Install Maverick

It worked for me.. no more sqare pixels !!!

Now I will tried to update to OSX el capitan from the app store too see if it's working by skipping yosemite

I have a mid 2011 IMAC

16gig of ram

I5

Very similar, if not exactly the same problem. Mid 2010 iMac 27". Ran hardware diagnostics and all it get is a temperature sensor. I'm struggling with that being the root cause as the behavior starts immediately upon login and the machine is not running hot. The behavior seems to correspond to screen updates from any application. Sometimes unplugging and waiting till the SMC resets helps. I cannot correllate system hanging to any particular function. Interestingly, the mouse continues to track across the screen but no applications respond.

Discussed this with Apple support online. They suggeted trying SMC and PRAM resets. Procedure for doing this is found online here: https://discussions.apple.com/docs/DOC-3603 The problem has cleared up on my iMac for the moment.

I've experienced similar problems with the advent of Maverick 10.9.2.
System self-crashes and restarts overnight.
Shimmering pixels present in the morning, accompanying the restart alert.

Running Apple's Disk Utility (permissions repair on internal 2Gb Seagate HD) resolves the shimmering, every time.
Any definitive test to determine if the video card is indeed responsible, and - if so - how/why does a permissions repair (explanatory reports differ, each time) resolve it?
Thanks!

My iMac 27 inch mid 2010 is experiencing the same issue. Out of Apple Care since August. I tried using Techtool 7 to identify the problem with no results. Funny thing I noticed is that I had to un mount my hard drive manually after booting from a Protogo thumb drive. Failed to unmount automatically, said other programs were using it. The issue also appears on the Windows side when boot thru boot camp. Finally took it to a local Apple repair shop. They replaced the video card but when I got it home I still had the issue. They now have replaced the logic board and still not fixed. Apple has been working with them through out the process. Not sure what the next step is. I've had apple computers since 1978 and never ran into anything like this. All programs and the OS are current!

Just to update my last post. Replaced logic board and still have the issue. Looks like a software issue but I have yet to find a way to identify the software causing the problem.

Just to reiterate, I've not had this problem return once since I adjusted my Energy Saver sleep settings to "Never" for both computer and display, and used a screen-saver... months ago.

I set both display and computer to never sleep and still have the issue. Must be something different for me.

I had this problem, even took the imac to an Apple service center and they could not find anything wrong with hardware. Afterwards I started stripping off peripherals and discovered that the problem was caused by the Logitech drivers asociated with my Logitech keyboard and mouse.

I've also adjusted my energy saver settings to "never" and "never". It has nearly fixed the issue, but every now and again, I will see the flashing dots appear for a few moments, but nowhere near the problem as before. The Energy Saver settings definitely have helped in my case.

I've adjusted Energy Saver to Never/Never. Problem is not close to what it was but I still see those dots. Not as many and not as often! I still wish I could figure out what the main cause was.

I set Energy Saver to Never/Never and for the last approx 24 hours, I have not seen the pixelation on my screen....thanks for the tip Citiboy. Prior to this, it was pixelating quite often and especially when I opened iPhoto, it went crazy. Now even with iPhoto, I dont see the issue. Lets see if this totally fixed the issue or if it comes back. If it does, I'll provide feedback here.

I have this same problem on my 15" Macbook Pro as well as the 23" Cinema Display attached to it. I took the MacBook to the Apple Store. They checked and said nothing is wrong with the hardware and said it must be software. They recommended first uninstalling and re-installing Adobe CS6. That didn't work, so their next suggestion was backing everything up and doing a clean install of Mountain Lion and all my software. Near the end of installing Mountain Lion, the lights started flashing again. It made me wonder if there's a problem with Mountain Lion installed on the Macbook Pro, but according to this post, the problem is not just Mountain Lion and not just Macbooks. So, I'm stumped and frustrated. Apple, please resolve this!!