Andrew,
Another possible culprit is your POWER SUPPLY. Are you using the cheap-o PS that came with the case? A study at Tom's Hardware demonstrated that many power supplies fail at WELL BELOW their rated output! Unscrupulous manufacturers exploit the fact that there is no user-friendly "benchmark" for power supply output. Many folks who curse Bill Gates' name when their Windows box crashes on them should look at their PS instead.
Fortron is a reliable brand. Their PSs keep running even at 80-90 watts ABOVE their rated output. (not that you should test this theory at home, kids!)
Culprit #3 would be the chipset on your motherboard. You don't have a VIA chipset, do you? Their drivers are notoriously unstable. Updating to the most current drivers
may help. Switching to a motherboard with the Nvidia "Nforce" chipset may give you better results. I've heard that the most rock-solid setup is an Intel CPU, Intel MB, and Intel chipset.