black & white squares? video card bad?

i have PC x800gto and the only game i play is world of warcraft, and after about 30mins in game, the screen will go crazy with these black and white squares everywhere, and i cant see anything, the ATI GPU crash thing doesnt come up or doesnt restart. it only does it when i play wow, i tried different drivers newest from ati, and old one, and ones on the disk all do the same thing, i cleaned out the case of dust. any idea? :?: thanks