I read all the posts about link pages dropping to PR0 and pondered everybody's theories. I would have posted to that folder but it appears the folder is full or something and I don't know where it continues.
Anyway: Some of my link pages dropped from PR4 to PR0 (white bar) in the last couple of days. The rest of my link pages stayed at PR4. Here is what I think the problem is:
I have about 6 or 7 pages of links. Some are called "links-location.html" and others are called "link-location.html." Most of these pages link to each other with an address bar at the top. Only 3 of these pages are linked from my index page (with a PR5). Most of the pages that dropped to PR0 use the word "links" while the pages that had the word "link" retained their PR4. I thought this was the reason they dropped but it was only a red herring.
Then I noticed one link page called "links-location.html" that had kept it's PR4. It is linked to from a second level page in my site that is also PR5. I then realized that all the link pages that dropped to PR0 were pages that only had links from other link pages with PR4.
Ordinarily I would have expected the link pages linked to from PR4 pages to have PR3, but it seems these pages receive no PR from these pages.