Flickering pixels on iMac

Random clusters of flickering pixels were a problem on my iMac (27-inch, mid-2010, Yosemite) for a few months. They'd get so bad at times, the system would freeze, requiring a hard shutdown. Opening new finder windows or documents and covering most of the screen with them would alleviate the pixel problem for a bit but it never went away. I was sure the graphics card was going bad. Then I stumbled across a solution elsewhere online that worked: turn on Reduce Transparency.

Here's how: Got to System Preferences and select Accessibility. In the Accessibility pop-up box, select Display and check the box next to Reduce Transparency.

I don't know why, but that has completely eliminated the flickering pixels. Saved me a trip to Apple Store too. The dock is now a solid color but that's a small cosmetic trade-off for a stable system.

I've had my iMac checked at the local Apple Service Center and they did not find anything wrong with my iMac. It's NOT the graphics card, they've tested it thru the night with intense gpu tests and nothing came up wrong. They couldn't even reproduce the problem as I'm seeing it. Perhaps it's something that is connected to my iMac that's making this? No clue at all! I've changed the ethernet cable and it has drasticly reduced the spots, but they're not completly gone. Image below is reduced by 50%.

Apple Hardware Test doesn't detect graphic card or display problems, but in your case, it looks like the graphic card or display are damaged. Reset the SMC > http://support.apple.com/kb/HT3964 Also, do a PRAM reset > http://support.apple.com/kb/HT1379

If they don't work, make a backup of your data with Time Machine and take your computer to an Apple Store or reseller. A graphic card would be cheaper in your case

I can not believe that this worked. Thanks so much!!! I was getting ready to get a new iMac.

I get exactly the same thing. Comes and goes. Typically shows up after working for awhile, with several apps going. Gets worse. Sometimes the system locks up or the display gets so bad you can't see to shut down. A restart usually clears it temporarily.

Mine looks like this from time to time, sometimes they're yellow and in the upper part of the screen. 😐

FWIW, have a look at > Apple computers: Troubleshooting issues with video on internal or external displays

My AppleCare ended in November, real bummer. :( But, I might be able to use my home insurance instead because it has a four year protection for electronic equipment. :)