I have a 4-month old website that uses an Exact Match Domain .net domain name, and it steadily moved it's way up to #5 for the search term that matches the domain name. It hit that ranking about a month ago. The progress was consistent, and it seems like Google loved the site.
Then on May 22, it dropped to #35. Bam! Just like that. Since then it has been bouncing around from #20 to #50 and it's around the high forties today. I don't believe Panda hit it as the pages are original content and while there is an adsense ad on each page, it is not spammy. But I found the following issues:
1. Back on May 22, I found in Webmaster Tools that there were many dozens of Not Found pages that got indexed in the SERPs. These were the result of a plugin that spawned crazy URLs into it's plugin folder from about a month earlier. I've since removed the plugin and the URL's from the SERPs through Webmaster Tools.
2. Then to make matter worse there was a DNS settings problem on for the nameserver this domain utilizes. I never saw the problem because nothing was noted in Webmaster Tools, and I never once had an access issue myself. But it turns out that some/many people did off and on. This problem has been fixed as of two days ago.
3. And to top off this comedy of errors, I had a robots.txt issue that was blocking the stylesheet (I had disallowed /wp-content/themes after finding issue #1 and disallowed too much), which was corrected yesterday! I found this with WMT's Instant Previews.
I've spent the last few days tearing everything apart and don't see any further issues.
I am hoping the problem is that even though Google did not report any errors on it's crawling ability (the DNS issue), that this was in fact the problem and downgraded all pages due it the inconsistency of the connection. If this is the case, I'd like to know how long until Google realizes that things are fine now and unpenalizes the site? And, is there any way I can accelerate this process?
I'm happy to send the site link to anyone who would need to see it.