I have a site that only has 30 pages, so it's easy for me to check to see which ones are in the Google index by using the site: operator. For the past few months, I've noticed that which pages are indexed changes almost daily. There are usually about 25 out of the 30 in the index, but which pages are missing constantly changes. A particular page will drop out for a few days, then reappear, then another will drop out, etc. This can happen to any of the 30 pages, and they seem to be chosen randomly. When a page reappears after dropping out, it usually returns to about the same ranking for its main keyword as it had before. All of the pages on the site have about the same amount of content, and all are interlinked with most of the other pages on the site, so that isn't the explanation. This site is more than two years old, so all of this should be stable. But instead, these random adjustments have been going on for months. I don't see any logical explantion for this kind of behavior. So I'm beginning to think that it is due to some kind of glitch in Google's database of indexed pages that causes temporary accidental dropouts of random parts of the database.
maybe someone is duping your content and its getting posted on high rank sites, then as it moves down the high rank sites pages your pages are more relevent to google??? maybe