I've had a bad PR spanking.
I think as my site goes across two domains, both intrinsically interlinked purporting to be the same site.
Ever since I launched the 2nd domain [just added a single letter on domain, most don't notice site is technically 2 domains it doesn't appear as such] my PR crashed.
Oh well - site still holds authority status in Google for about 3 combos of the main keyword.
Annoys me, but I can't stop the interlinking, so I am doomed IMO