Blinking Pixel error

Hi

As you can see in the picture Im having a pixelproblem of some sorts. I get these dots regulary. They blink rapidly and disappears somtimes when i change desktop or open a program. The end result though is always the same: The whole screen freezes and lastly the mouse stops working and I have to shut down manually. Ive had the Imac examined by a company. They came to the colclusion that I had a defect RAM. I got rid of the old RAM and bougt a hole new set. It hasnot helped as you can see.
I have also tried to clean install Mac OS. It took a long time before the dots appeared, and for a couple of days, I though I was saved. But they came back.
Has anyone experienced somthing similar? - or know what is causing these dots and what to do about it?
I am at a los. I have, of couse tried to search the problem, but to no avail.

It looks like the GPU is failing in your iMac. Though, curious that a re-install would get rid of it, but it's got nothing to do with RAM, so whichever company you went to doesn't know what they are talking about.

How old is the iMac? Do you have AppleCare on it? Is there an Apple store nearby? If yes to last two, then take it to an Apple store promptly, if it's an older iMac with no AppleCare, still take to the Apple store to get it properly diagnosed, and if it turns out to be a logicboard replacement, expect it to run in the $750+ range..

My mac is from 2010. I was fearing it had something to do with that..

Well 2010 is not bad, did you purchase AppleCare at that point? You MIGHT just be within the range if you did. Otherwise, these kinds of visual defects, tearing, spots, and just all out bad video is usually indicative of a failing video subsystem.

Unfortunately, on the iMacs the video subsystem (GPU) is part of the logic board, so full replacement is the only way..and being a major component of the iMac, it's quite pricey..