Could it be same anchor text from 500 sites regardless of what sort of sites they are?
That certainly looks like what is affecting me. There are two search terms - one the main money term, the other a technical help term - that are in high quantity backlinks (none from directories, and none of them solicited), and both terms have gone from top of page 1 to pages 4 and 5 respectively, with a downward lurch of 5-10 places every week or so.
The penalty applies to exact match of anchor text (so widget is clobbered, but movable widget isn't).
Other terms (including ones that find those pages) do not seem to be affected.
As I have already mentioned, the periodical drop makes me suspect Penguin is recursive, and probably applies to exact match anchor text.