I see this jumping too. For like a few hours page 1 then reverts to page 3 and may be there for days. It is like there are multiple sets of results. That might be the reason say they think there is a switch they turn on and off. Just depends on what set of search results they are serving up.
Personally I am getting frustrated at how slow Google is updating information. If they are going to ding us on backlinks, lack of nofollows, and all the stuff in WMT like short titles and robot.txt then they need to be indexing faster or not count it. Google today is surviving on their history (what they provide in the past) and not their current results. They don't seem to get that their little tweaks are killing traffic for many good companies and for the end user. If I were Bing I would spend millions on an ad blitz here in the next few weeks. That could cripple Google because right now Google is not providing quality results.