I've had time related freeze problem twice before on different computers. The first time, the CPU fan was getting a bit old (3 year I think) and it wasn't running perfectly. My computer kept freezing when I used CPU instense applictations (such as games) for more than an hour. Gradually, the problem got worse, and in the end I couldn't start up my computer for more than 15 minutes. Took a while before I realized what was really going on.
The other time, it was my hard-drive that failed on me. But this was easier to detect because it made a screeching sound before it froze. I lost like 60Gb of data that day...
Tovi 
