I have had the same problem with a similar site. I have been in the rankings on Google for hundreds of related keywords. My rankings just jump around. One day I will be number 20 for one of the top keywords. Next day I will be 200 without doing anything at all. A couple weeks later I jump back up again. With a highly competitive niche it's hard to hold a spot for long. I spent 2 years at number 1 on MSN and Yahoo, but Google just bounces me all over the place. My site has been up since 2005 and it seems like I get thrown in the sandbox every few months for no reason.
reading to all these comments making me curious. Actually which one is the worst, banned by google in their SERP or dropping your PR? Because currently I drop my PR but i checked I still manage to get some keywords on strategic pages.