I have one important page that has been popping in and out since August. No errors in WMT. PR4 as far as the toolbar is concerned. Tons of links to it. No reason as far as I can tell. It's a redirect (example.com/page.html redirects to example.com/word-word/) but that redirect has been in for at least three years and this only started happening in 2010. Right at the moment, it's back in and ranking high. I have no explanation, just marking it as yet another Google mystery. The content on the page is dynamically generated out of a database, and I have done NO tweaking on it whatsoever.