zerillos - 1:26 am on Apr 9, 2011 (gmt 0)

This article has a list of other potential reasons your site may not be doing well in search.

what's the list?

site may not rank well if your server stops serving pages to Googlebot, or if you've changed the URLs for a large portion of your site's pages.

did you check this? it looks important.
does your server, serve, the right pages for a particular URL?