mctaino,
I'm assuming that since you have put new compound on the heat sink that it is seated properly, the fins are free of dust and the fan is working properly. If that is the case there are a few things I would try.
The most likely cause for a machine acting like you describe is a failing power supply. Try picking up a 400 watt or so power supply, install it and the problem will most likely go away.
The second thing is bad RAM, especially if you have onboard video or a card with shared memory. Try swapping the chips around and make sure they are properly seated.
The other thing I have run into that caused behavior like this was a failing video card. I know it sounds weird and I would have made a funny face if someone had told me that before I experienced it myself.
Tracking this kind of error down can be a pain because everything works right up until the time it doesn’t and when it doesn’t it can’t tell you where it hurts. Good luck and may the force be with you.
