Dropped site checklist?

Any kind of checklist available to help diagnose reason for being dropped

[brmmbrmm]

I have a site that was set up in March 03, and listed nicely on Google. Around September its position seemed to deteriorate, and then fell of the list.

Now it ranks nowhere for any search, even obscure ones which would only appear on this site. Yet it is linked from a PR4 page and Google's cache for this site is updated monthly. PR for this site is 0.

I have been worrying about this for 3 months, but Google's "funny turn" doesn't seem to be coming to an end. I've checked for HTML, and it is pretty sound. There's no cloaking. No hidden text. It does use dynamic URLs but in the same way as other sites of mine which are doing fine.

Does anyone have a checklist for what to try in situations like this?