I had a problem once with my 6600gt, it drove me nuts for ages
.. and i tried various drivers both in win98 and win2K, and it didnt resolve it...
My main problem was the card was stopping/freezing every second for a split second within games.. and the videos/mpg in the menus wera also doing it sometimes with corrupted graphics.. mainly nfs1.. it turned out to be my USB wireless adaptor causing the problem trying to find the other one.. try taking out usb peripherals?? maybe this may shed some light on the issue?..