Funny you should post here... I also get random and disastrous crashes periodically. First I thought it was a cpu halt because of temperature, then a software crash something to do with Opera but basically I have no idea what does it. The machine hangs, no messages, no option but to hit the power button. With SuSE Linux fsck has managed to sort the hard disk out each time (although it takes a while) but it is a pain in the a__e.
Looks like a hardware problem to me, but no idea how to go forward and isolate the cause. Do you use the onboard VGA?