>It also sometimes seems to hang when you leave the computer for
>a while. I turned off the screensaver but it still hangs when
>left alone long enough.

An idea. I remember a strange similar behaviour I encountered when I was a student. We had 12 identical Pcs at the school, and one of them presented the same symptoms.
In fact, it was a bad soldering contact onto the mainboard. While cold, the PC worked fine. But when hot, after a few hours, due to dilatation, the solder did not get in contact and the PC hanged up sometimes. Sometimes because it appeared only if the monitor was placed onto the CPU case causing box deformation ...

Excuse me for the delay but I was on holidays.