Hi strikelit,
You ask "What else could be causing this?"
I'd have to initially answer anything from a faulty CPU fan (causing ythe CPU to overheat and cease) to too many apps running at once and RAM being full, full HD from SWAP, etc...
You may want to give us a bit more info like; what you running at the time of the freeze up, did you do a hardware check to verify RAM and fans and the like are not faulty, what do the syslogs say at time or even leading up to the event, etc...
Good luck,
DrD