That's been happening recently with my sites... It's called the "Google dance" (a technical term)... It basically means that google is tweaking with its algorithm... THeres not much you can do about it, but it will probably settle down in the coming weeks.
The google dance has been a somewhat frequent topic of late. Check for the other threads and you might find some additional insights beyond HazeyDaze analysis, which is sadly on the mark. If only there was some way that the SEOers and web designers could make a statement to google, sort of like and online strike against google, it might send a message. But it is much easier to theorize then to put into action.
Google in the last 3 months have been doing allot of changing. They have been running monthly PR updates for the last 3 months, and in turn they have been tweaking their code, I have noticed at least twice a month, my sites heavily flucuate in the SERPS often for the worse, then they go back to normal when i re-double my link building. I think its just "normal" but then again, it could be your competition getting more links when the cache updates, your links expiring, and so on.
A really obscure keyword/trade/business/target-audience or one is a small geographic region, is easy to keep in the high ranks, there arent many other players on those keywords (fortunately this is me, no competition)
A struggle for market share in a larger region, or market, or keyword/trade/business is going to get bumped around as other sources of the same keywords spider, or other sites change their keywords and SEO strategies. (this is you, heaps of competition)
Poker and pay day loan are the topics which have very high competition so you may need to work constantly, this fluctuation is commonly found in this category. to decrease this variation you must increase your backlinks.
Just a note: If your site is penalized, you might get hit with a +50 penalty. So if your ranking drop, and don't return, check the difference in ranking... If its a flat number like +50 or +100, that's evidence of a penalty... Otherwise, it's probably just dancing...