I ended up needing to reboot my computer today, so I did one pass of a MemTest (with success), but don't currently have the ability to do it for a whole day, due to the nature of my work on the computer (although this might be possible about 2 weekends away). Anyway, just after doing that, I noticed that the 2.6.25 kernel was in Debian, so I changed to that and the problem seems to have resolved in linux-image-2.6.25-1-686. David