I had an XP 1800 . It would lock up at anything above 50oC. I wondered what the hell was up for MONTHS till I cooled it some more. Reinstalled windows, drivers etc etc not believing my coolermaster case etc could be too hot. It was. I went through hell! Watch those XP chips. I think some of them don't like hot at all!
Jeez. I learned if you get hard lockups at all, look at heat first.