You Are Not The Only One Doing SEO

If you experience a dorp in SERP for a few or more place/pages Its not always http://dance.efactory.de/or http://en.wikipedia.org/wiki/Sandbox_effect (check the links for info about those terms)

Just remember that there are 100s or 1000s of webmasters targerting the same keyword and are also building links for there webpages.

If you beat a few sites and finally sit on a good place on the first page dont think that those sites will not fight back to gain there places