Eld's suggestions seem right on target. My first thought is overheating, although if WoW works for long periods of time, that would poke a hole in that one. Regardless, be sure to check the temps in your computer: processor, video card, power supply, etc.
My second guess would be power supply inadequacy, but that is also weakened by having WoW work normally. Have you installed any new hardware that didn't come with the machine? E.g. hard drives, cd/dvd drives, etc. Do you know what the wattage is on your PSU? If you're not sure, you can open the case up and have a look at it; there should be a sticker with the wattage and some other info on the unit.
If all else fails, I'd try a reformat. It can be a pain in the ass, but imho it's worth doing periodically because it just keeps things running smoother.