For me, the big fluctuations in traffic quality are in the longtail results. These are really tough to track because some of these queries might not even get typed in to google twice in a week. But clearly there are lots of sitewide trust factors that affect longtail results - I call these voodoo factors because most of us and probably very few google engineers can even understand them.
My two-word keywords that go to the home page, class pages and category pages have been relatively stable and they don't seem to move when I am experiencing swings in quality of google referrals.
As for today, we slipped back into voodoo zombie land a couple of hours ago. Today's better converting swing, though, wasn't quite as high quality as some of the highs that we have been seeing. Someone, or possibly some machine, is still assuredly busy turning dials.