Steam: Lockup, forced to restart computer

[Hypatian]

Hrm. I'll keep trying, but this looks like it's not going to work well to spot the problem, if overheating is in fact the problem. I played a new game three times. The first time, up through killing the dragon in training. The second time, up through killing the necromancer. Neither time did the temperature get very high (a bit lower than max playing Team Fortress 2, in fact.) The third time was going to be through the treasure hunt, but the system crashed at the very start of the necromancer fight. I have no idea what the temperature was then, but it was just fine after I rebooted, so... *shrug*

Since the problem appears suddenly and without warning, and prevents any useful recovery of information after the machine needs to be power cycled, I'm not sure what to do.