yay :-) By the simple expidient of removing one of the two DIMMs, I seem to have cured my powerbook. Sure, it’s got 1/2 a GB less memory than it did, but it’s still OK for most stuff. And it stays up for more than a quarter of an hour at a time. Joy.
Update It locked up after about 2 hours. So that’s not it. Boo :-(