THis may sound strange but take it from someone that has done it for real with success. If after everything you try by conventional means the graphics card still gives such artifacts/corruption then as a last ditch attempt try baking it in the oven!
I have a 8800GTS 640mb card which had black and white flickering lines over the screen, unintelligible display. Did some forum hunting and came across a guy on youtube that baked his card with success. Do some research yourself and you will see that i'm not telling porkies! In my case I baked it at 185C for 8 mins. Be sure to remove the heatsink and any plastic you can, I baked mine upside down and had alu foil underneath to catch any solder if it dropped so it wouldnt ruin my oven! Anyway, I'm now sat behind a 24" display writing this thanks to my now restored 8800GTS.
Did I think it would work? Hell No, its crazy right? Did I have anything to loose? Hell no, it was buggered anyway as far I was concerned! Was I surprised to see a perfect picture and a booting PC afterwards, hell yeah! Sorry OCuk but no need for a graphics card yet!
There was a thing sometime ago about people wrapping up their Xboxes in a blanket when they had some equivalent of a BSOD and corrupt display. Essentially it was the same method although was highly ridiculed due to the average (don't slate me, I just prefer PCs!) console user unable to think beyond their joypads!
The theory is that under normal use electronic pcbs (and hence their solder) develop microfissures in the structure. Reheating it can re-flow the solder without melting it completely.
Those cards had naff coolers and generated far more heat compared to the current generation of cards. Can you try the card in another machine at all?