OK, here's my?: I have a site that is about 2000 pages. About 90% of the site are dynamically generated .php pages. Up until the end of February it had about 1350 of them indexed. I have keywords I check weekly and do a site: search. On Feb 28 my site dropped 600 pages down to 750 indexed. This has happened once or twice before but the pages always came back after a few weeks. It's been a month and a half and they're still not back. My rankings really improved after the last update but all these pages were lost. I did update almost all of the title/meta tags, but that was back in September(?).I have noticed that some of them have gotten tagged with the "supplemental" result(with the old tags) even though they used to be indexed properly with the newer tags. One of the pages that gets a lot of traffic from google just dropped to a URL only listing where last week it was ranking top 3 out of about 150000. I changed the page some but it was only to add more content. I've read about the dup content issues but I try to make sure that there is at least 40% different content on all pages.
Any ideas of why some many pages are being dropped and if it's something fixable?