I've been studying page 2 and page 3 for a while - and the rankings are incredibly volatile. It's just not like the "good old days" anymore. You used to watch a URL slowly climb (or fall) from one spot to the next, as the algorithm rated all the factors and changes.
But today, Google apparently runs statistical tests on user response to various types of results - especially in the deeper pages where impressions and clicks are very low. In fact, two computers side by side at the same location can get different results, especially in the deep pages and on long-tail query terms.
Google is trying to learn something from these changes, and I'm sure there are complex patterns that are hard for us to see. But there is one thing that we should learn. We can no longer take any meaning from position changes in the deeper pages of the search results, and it's not worth worrying about. Until your URL climbs pretty high - and how high seems to depend on how competitive the term is - unstable rankings seem to be the way it works today.