I have a very basical informational site with about 50+ pages. Specifically, there is a page about each site. I have been tracking pages daily for months and they have been streadily increasing in the SERPs. Suddenly, about 3-4 weeks ago, about 20 pages were completely missing from the SERPS, though they were still indexed (I could google a specific text from the page in quotes and it would appear). Since then, pages have been added and removed from SERPs continually, cutting my traffic by about 50%. All I have been adding is information about the sites topic. I haven't stolen content or done anything else that seems like it would cause this.
Rankings are not static and can change without obvious reasons. It doesn't necessarily have to do with anything you've done. There are plenty of off site factors that can affect this or Google could just be making internal changes that are causing fluctuations in rankings.
All you can do in a situation like this is continue to improve your content and acquire quality links to your pages. If your content is good this will ultimately work itself out in the end.