OKay, I think I may have found the root of the problem: Heat
I was stress testing my comp using Prime95 (thank you jolancer) and set the time for about an hour, just for a small test.
Went downstairs for a bit and left the comp to do it's thing. Before I left, I noticed the temps going above 70C (around 75-78 every now and then) I left and returned after about a half hour - 45 mins or so. I came to my screen saver and moved the mouse. Nothing. Looked at the hard drive light, nothing. Conclusion: Frozen.
I know that many sensors cause the computer to shut down if the limit has been reached, I am beginning to think mine locks up. It would make sense only because the test was running and the temps were getting kind of high. So it seems almost logical for this to be the reason for my lock ups.
What do you all think? Does it sound possible?