Thank you to all who offered suggestions. My machine turned out to have a bad RAM module. It's back with a new module and its CPU heat sink reapplied with fresh wax, and seems happy now. For some strange reason, after it came back from the shop, Ubuntu lost track of the USB bus (couldn't find the scanner, couldn't access the card reader, couldn't sync the palmtop) until I rebooted with a Knoppix 3.4 live CD, which found everything - only after that did Ubuntu find them too.
=====Andrew
