diberry - 2:57 pm on Nov 2, 2013 (gmt 0)
It also sounds like an extreme version of something I've observed in the past when Google is sending mismatched traffic to your site in droves. I've only seen this once or twice, usually on new sites, as if Google is liking your site but not sure which users really should be sent to it. I assume it's a form of testing.
However, I never saw fluctuations on the scale you're seeing, so that might not be what's going on in your case.