tedster - 9:05 pm on Apr 3, 2011 (gmt 0)
Weve been kicking this kind of ranking strangeness around for a while - at least three years. See Yo-Yo Effect - Observations and Understandings [webmasterworld.com]. It now seems to be more common than it used to be, especially the past couple months.
My suspicion has long been that this kind of yo-yo ranking is an artifact of Google's machine-learning algorithm. In other words, it's a kind of testing and learning that goes on automatically. If I'm right, then one of the things you might learn if the up-and-down continues for too long is that your site's signal is too confusing for Google to evaluate.