Sudden Drop in PR from Bad Linking?

[DXL]

My company site has about 30 pages of unique content related to our services, and about six resource pages for link exchanges. The homepage had been PR5 for years, almost all the interior pages were PR4.

I just checked out my site and discovered that the PR value of all my pages have dropped to PR1-PR2, save for my contact and home pages. I care more about my position in the SERPs versus PR value, but the drop in PR on the resource pages means most designers will be less likely to trade links now.

What is odd is that I haven't exchanged links with anyone else in months or maybe even a year. I've got inbound links by way of site credits for new projects, and I occasionally link back to some of those sites for my portfolio, but they aren't in bad neighborhoods. So how could my PR have dropped so much, and what can I do to get it back?