Have you tried the different options in
Preferences/Specify thumbnail quality
to check if the problem is limited to a specific mode or generic?
Yes, I tried always high quality thumbnails, high quality on demand, and 100% previews. The problem persists. Flushing the cache normalized the thumbnails, but as the cache is rebuilt the problem recurs
If nothing else works, I may deactivate CS5 and reinstall, but that is a bit of a bother.