It's NOT overheating. Tons of people with Radeon cards are having these problems.
What drivers are you using?
Have you tried latest Omega / ATI drivers?
Yes.
Same problem, very bad polygon spiking all over the place.
Sapphire Radeon 9500 Pro 128MB
P4 2.4GHz HT
4x 256MB Corsair PC3200 DDR @ 400Mhz
Asus P4C800-E Deluxe
SB Audigy2 Value