MaxFixIt is reporting that RAM slots are failing in some PowerBooks:

We continue coverage of an issue where the lower (or less frequently, upper) memory slots in some PowerBooks fail, leaving users with less available RAM than is actually installed. As previously noted, some users are not aware of the issue until they check the "About This Mac" pane accessible via the Apple menu or experience significant slow-down in a particular application.

It now appears that there are two causes, and two very different solutions to most cases of this issue.