How often do you update your site? PR can fall for a huge number of reason and not updating for a long time is one of them. Your site may be a victim of Google Dance and the panda who filters the content may push your site from first to last. So jut don't get worry and optimize your site with a new enthusiasm.
I think it's just Google dance, if you know that you are doing the right thing, and your ranking still dropped, don't worry it's just a normal thing.
This kind of stuff is often expected. Just keep building links and all should be okay. Like everyone has been saying- it may just be doing the 'dance'.
This is bound to happen. You need keep on building the quality and themed backlinks constantly so that you are in the race
There are several reasons for ranking fluctuations like, - Your rankings were influenced by personalized search - Search variations (Mozilla/Virgin homepage searches may give different results than Google.com) - Or the first time you checked your rankings, you were logged into your Google account. For the Searching - you can make sure you always check the same way... * Logged out of G Accounts * Opted out of Personal Search * Use the same computer * Use the same browser * Use the same Search page * Use the same Search settings
It could possibly be that one of your main links has been penalised by the search engines, or your link has been removed from the site, and the search engines are now re-evaluating the search results, or it could be you have fallen under the Google duplicate penalty
Don't think about of anything just do what you do best. Currently experiencing it also. From page 1 to page 3 on my main keyword. and page 1 number 1 to page to on my second keyword.