Mysterious little squares on my iMac

[Michael Verhaeghe]

Is anyone familiar with the phenomenon whereby the screen of an iMac 27" is often dotted with hundreds of colorful little squares, in certain patterns. They come and go, and they slow down my computer significantly. I have been to an apple dealer and they checked up my computer but could not find anything. The graphic card and the HDD were working OK, I was told.
I attach three photo's that I had to take with my iPhone : each time I tried to make a print screen (shift command 4), the squares disappeared and a soon as I had clicked the print screen, they came back ...
Very annoying indeed.

 

[LoftyHammond]

Might be a dying graphics card...If it's still in Apple warranty, get it checked out

 

[antman2x2]

Yikes... GPU issue for sure.

 

[zerocustom1989]

Welcome to hell. I have this problem. Sometimes it causes my Mac to freeze, and I need to force-kill the machine to get it back.

Been to the Apple Store twice, both times they said they couldn't detect any problem with the machine, and wouldn't replace any parts in it, even though I insisted they do so. My mac is out of warranty and I've just decided to live with it and put the money to a new machine.

For me, this issue happens more if the machine is cold. I think there's some broken solder joints on the logic board that open up as the board flexes. If you need to keep using the machine, try and force it to run hot for a few hours, I noticed that buys me a few days/weeks of time before the squares reappear.