Joined: 05 May 09
20 Sep 11 1:00 am
Sometimes you'll see relatively new sites rank highly for awhile then drop away for a few weeks/months then go up again. This is because Google is re-evaluating the site to see if it should remain in their index and what keywords it should rank for.
Don't worry about the those sites, they would drop in the rankings if they have low quality.
Just make sure your site is properly optimized. Keep building quality backlinks and add fresh content to your site and you'll eventually see your site on the top position.