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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.
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.