Visual errors on 2011 iMac

[Billydelp4]

my 2011 iMac 27" 2.7 i5 is having some issues with visual errors. Last week it suddenly took on a yellow blocked visualization, which required me to hard shut down since I could not make out the visuals well enough.

Now this week I am seeing blue and purple blocks pop up in corners of the screen. I have attached a screen shot of some of the blue blocks that I saw today.

I have repaired disk permissions and they seem to have disappeared for the time being but I am afraid there are more underlying issues that will ultimately come back.

any advice on other remedies or known issues would be appreciated.

Thanks