I had similar probs with my 8500 pro. Every so often, maybe about every 2 weeks, BANG, here have some lines all over your screen, and hey, how about some bizarre shapes!
I tried everything! TWICE. Anyway, 'bout 6 weeks ago I replaced my shitty 300w generic PSU with a rather splendid Antec TruePower 480, as I upgraded some components and though it was a good time to get rid. As a by-product of the PSU upgrade, the problem has decided to f*@k off. I've never gone this long without the prob. Seems to have worked for me. Maybe this'll give you hope!