Do you have OFB? I used to have this problem, but I updated the drivers to play Oblivion, and the problem doesn't seem to exist for me anymore.
At any rate, this has convinced me to buy an ATI the next time I got a card.
Take it from a long time ATI user, stick to Nvidia... Even with the nonsense with the colors flashing, it is a FAR better card. Shows bump mapping perfectly etc.