I think I mentioned before, for you I’d seriously consider going through all your paid links (like the banners on the left) and adding a rel=”nofollow” to the link tags. Pagerank works mostly by looking at how many links there are to a site and considers each link like a “vote”. Adding a rel=”nofollow” tells Google to not count that link as a vote.

Google went a bit mad recently and decided that by not adding a nofollow to paid link, you were trying to “cheat the system” as people could pay popular websites to link to them to make their pagerank better. Thats probably the reason why your pagerank disappeared so quickly.