It crashed again, I rebooted, it got to the login screen, and I typed nearly 6 letters of my password before it crashed again. Uh oh
So I have reason to suspect the case is fucky (grounding problems with the USB), and this feels like either a motherboard or cpu problem.
The cpu is like 5 years old, the motherboard only 2.
Could also, in theory, be ram?
So I'll need to memtest it, and also try watching the temperatures while it runs. I've never had cooling problems before, but the way it kept getting worse when I rebooted it smells like cooling.
@foone I had a desktop PC that spontaneously powered off when it got a bit cold (~65F, not that cold). After fearing all sorts of dire and expensive problem sources, it turned out to be the case front panel, and disconnecting everything solved it.
(I set the PC to 'always power on when power comes back' and got a little power button on a cable, just in case. Fortunately I didn't use the other front panel stuff, although much of it is probably safe.)