I noticed some of my sites dropped like this too, after the update.
What worked for me was changing the anchor text of my high PR links (since I have control over those).
I believe that the new update now penalizes over-using the same anchor text for your links, even more than it did before.
Once I varied the anchor text of my high PR links quite a bit, and starting building new links with anchor text varied quite a bit more, my rankings came back on most of my sites, if not all.
Just a suggestion.