tedster - 6:51 pm on Aug 31, 2012 (gmt 0) [edited by: tedster at 6:55 pm (utc) on Aug 31, 2012]
I first saw this kind of ranking behavior in 2008. It was for a household name website in a very crowded market with two synonymous keywords - and the rankings began to pop all over the place right after a big online marketing agency (not mine) began to work with them on-page, on-site and off-site.
They went from page three to #4 then #16 and so on. They ended up at #4. The ranking fluctuations only stabilized when the SEO agency STOPPED making changes to improve rankings, but that new stability took months. And we are talking about a very big international brand, here.
Some of the fluctuations were seen several times within the same day, but others only showed up as trends over an extent of time. Sometimes how you watch the data is as important as the fact that you watch it at all.
[edited by: tedster at 6:55 pm (utc) on Aug 31, 2012]