Google does not de-index websites unless they are violating the published Webmaster Guidelines.
The good news is that if you remove the offensive content you can get your websites re-indexed. I recommend that you study the guidelines and fix what is wrong with your websites. It’s all about providing a quality user experience, do your part if you want to be included in Google’s index.
This sounds like the typical results you might see from QDF (Query Deserves Freshness), Google’s freshness factor. Once this temporary boost wears off, your page will sink in the rankings to it’s truly earned rank.
Since different keywords will have different levels of competition, how far and fast your page’s rank fluctuates will vary based on that competition.
There are other possibilities, like someone scraping your content and outranking you with duplicate content. But most of the time it’s just QDF.
It’s Voodoo SEO, you have to believe in the curse… err… the penalty for it to have any power over you.
This sounds like yet another case of QDF. Please see my post above: http://www.warriorforum.com/adsense-…ml#post3682620