Agree, it can be a zillion things!
How about in idle ? When you're not doing anything on it? Does it do the same?
Do you get any error messages, any blue screens? Anything in Event viewer?
Try it in safe mode and see if it does the same...
Or try loading only basic devices from : Start>Run Type msconfig. Click Load basic devices...bla, bla... click ok and restart
If it happens only when U use it, then it might be temp-related. I use Speed fan to check temperatures. Download it yourself and post the temp!
It could even be a startup service or a plain service that is causing this...