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.