For several months now, people have noticed a problem where you can lose your SERPs ranking by being hijacked when another site uses a 302 redirect to your site. Rustybrick gives a good explanation of how this works when he said "GoodSite.com ranks in the number three slot at Google. EvilSite.com comes along and copies the content directly from GoodSite.com. Then EvilSite.com puts a 302 redirect from EvilSite.com to GoodSite.com."
Of course, many people have no clue that this is happening to them. A good way to check is to search for inurl:domain.com and then look for links that link to your pages using a php tracking script. The most common one is tracker2.php.
In fact, if you search for "tracker.php" in Google, you'll see an incredible array of spammy sites that mostly exist to show Adsense ads.
If you have lost your rankings, you might want to see if you have been a victim of this 302 redirect problem. And if you have, complain to Google and complain loudly! It is far past time for Google to do something about this.