Does it look like this? http://www.youtube.c...tch?v=ofuNEkncGQk

A few years ago there was a problem with certain video chips having bad solder bumps (due to a change of the solder technology to meet ROHS standards). I don't know if the card in your iMac is from that vintage, but it might be.

Hunt around iFixit to see what's involved and whether it's worth the trouble.

Can you plug an external monitor into it? If so, maybe you'd get lucky and that would work even if the main display was being wonky.

HTH.

Good luck!

Cheers,
Scott.