diberry - 2:45 pm on Nov 2, 2012 (gmt 0)
1script, I've seen very much what you're seeing off and on, and talked about it some in the thread Tedster links (even though I'm not sure it's zombies/shaping, but it seems to be related somehow - maybe).
Sometimes even just for half a day, I'd be ranking for one set of keywords, and then later in the day it would flip to a noticeably different set. My "usual" set is competitive phrases with just a couple of long tail words added on. My "alternate" set was much more long tail, but much more specific/accurate. The bounce rate and other user metrics were much better with the alternate set, but in the end Google settled back on the "usual" set.