Thumbnails won't resize in PSE 8

A strange problem occured yesterday: Thumbnails won't resize in the Organizer if I display more than 20 pics on my 24.5" screen. Fewer than that, everything works ok.
The catalog currently holds approx. 20000 pics. I also have the CS5 and the Bridge has no problem resizing. Problem is that I have tagged the pictures in PSE8 and therefore use it quite a lot besides the CS5. Adobe told me long ago that it is not possible to import tags from PSE8 into Bridge. Most of my pictures are RAWs of various sizes. Any clues ? (and many thanks)!

EDIT: I certainly have 20000+ pictures in the catalog and mk IV full RAW is big files. So I have set up a new catalog starting from july and we shall see if that helps?

There were something wrong with a file or two in PSE 8. I reinstalled an old backup (two weeks old) and now it is working like it should. My standard catalogue contains 20000+ pics, so maybe I should create a new one? What are your experience, if any, so far?

I haven't used Elements for several years -- when it does a backup, does it actually back up your images as well as its catalog funtions?