Netscape Disappearing Bookmarks

After my machine has been running for some timeI find that my bookmars window displays a blank pulldown menu. The bookmarks are still there; I can click in the blank areas and the bookmark will be opened. Also, when I move the cursor over bookmark folders, the folders will open and display the contents of the folder. I have looked over other discussion groups that suggest the problem may be due to a corrupt cache folder. But, I think that applies to only when the bookmarks file will not open. This is like the bookmark text and background color are the same for the main bookmark pulldown. Anyone have any experience with this?
(Win 98SE, Netscape 4.7, Pentium III 600MHz, 128MB RAM)
Thanks