I have a similar situation on my site. It's a PR 8 site that used to rank for just about anything I wanted. Then I added several hundred thin affiliate pages and all my rankings dropped from page 1 to page 4. I still rank #1 for my site name though. I also still rank high for specific searches in "quotes". I'm not sure what caused the penalty. I think I got a manual penalty but I'm hoping it's an automated filter of some sort. From other people's experiences on this thread, it doesn't look good for a revival.