It’s not the power supply. I just swapped it with another computer’s (a fairly new, 450W one) and the same thing happened.

Next is a memory diagnostic.

[Update at 12:50 PM EDT]

Memtest86 found five errors in the first pass, on tests 6 and 7. The problems with the Athlon supposedly show up on test 5 or 8, so it’s probably not a speed problem. I’ll let it run another pass and see if it sees the same thing the second time through.

[Update at 1:17 PM]

Only one of the errors repeated, on test 6, but it did repeat.

[Update at 2 PM]

I let the machine reboot into Windows after the memory test, and it’s now been up for over half an hour, which is a record for the last couple days. I don’t know why running a memory test would have that effect, but the problem seems to have been (at least) mitigated, at least temporarily.

[Update at 8:40 PM EDT]

OK, the machines is now up for over six hours, with no glitches. I’m posting from it. Go figure.

No, thanks, I know this isn’t the end of the story.

[Friday morning update]

The machine is still alive and healthy this morning.