Well it seems somewhere around the 20th my site has taken a nose dive in Google for my main keywords.
Trying to figure out why, I have noticed in Google WMT that it is reporting a whole bunch of pages not found. The weird thing is that they are all pages that have not existed for 6-7 years and were specifically removed by a removal request Eons ago.
Not sure what Google is doing right now, but they must have something really screwed up.
The Not Found report is for your information, it's not telling you those pages "should be" on your site or that a ranking problem is being caused.
However, it is possible that there are backlinks somewhere that point ot those urls. Have you checked that? You may be missing some PR power by not serving at least some content at that url, even a 301 redirect.
I found "The Not Found report" to be very useful on one occasion when it came up with a missing page that should not have been missing. I'd goofed up and missed recreating it when I did a major overhaul/redesign of the site.