Swanson - 4:20 am on Mar 15, 2006 (gmt 0)

To me it sounds like canonical home page problems.

Linking internally to "home.htm" can cause the "duplicate content" problem. Google sees your domain name and the "home.htm" as 2 separate pages with duplicate content and/or can not disseminate the links from the domain name through the site as none of the internal pages link back to the root (the domain name). It sounds strange, because it is. But that is what the Big Daddy update is meant to be fixing, unfortunatley it seems to be making it worse.

Either way, linking all internal pages to the root domain and replacing session URLS with another form of session tracking like cookies would seem to be the best change you could make if any.