I had this with my GeForce 7900 when i tried running Oblivion on max settings. Once it's heat damaged it's heat damaged. You can do things to slow the eventual degregation, but the longer you leave it the more of an inconvinience it'll be when it finally croaks; and the smaller the gaps between it causing polygon/pixel artifects.
I'm sorry to say but you need a new GFX card.
I'm sorry to say but you need a new GFX card.