RADEON 9800 Causing Reboot/Overheating

While playing games (both Doom 3 & Painkiller) my system will suddenly
restart. There's no image degradation before this happens. It just cuts
out. Using AIDA32, once I'm back in Windows, shows the GPU temperature to
be around 60C. CPU is fine in the mid 40C's. I've added additional fans to
no avail. Spec is :

Intel P4 3.4GHz
DDR SDRAM 1Gb
RADEON 9800 256Mb; CATALYST 4.10 drivers
On board sound VIA AC97
HDD 200Gb
Power supply 650W
Fans - 3xfan power supply; CPU; GPU; 80mm to rear; 120mm to front; exhaust
fan by GPU
Other drives - DVD R; DVD R/W; floppy
Windows XP SP2; DirectX 9.0c

Anyone had similar problems? Did you resolve them? All help gratefully
received.

Many thanks

M