Sgt_Kickaxe - 5:17 pm on Dec 16, 2012 (gmt 0)
Curious, what does GWT tell you about your sitemap, if you have one?
submitted = 1000 pages
Indexed = 900 pages (10% fewer indexed than exist, for example)
Sometimes the above is a sign that a spammer has been busy finding pages that resolve to incorrect urls on your site and is linking to those, which causes the wrong version of your pages to be indexed. Wordpress? Check for urls ending in periods and shortened urls.
examples for: example.com/a-perfectly-valid-url/
Check header responses. Bad = automatic redirect creations with infinite variations possible. Worse = showing the right content on a malformed url without rel=canonical at a minimum.
Test to see how your site reacts to these types of malformed urls, if you see content from the correct version but the wrong url or a redirect from malformed to correct there is a chance your rankings may be impacted.