I think Google is weeding old or stagnant pages out of the index to make way for new pages, it is the only way they can keep up with the internet IMO. I recently did a search for a topic from 2002 and it was like going back into the stone ages in search. Everything now is what is happening today, not years ago. I don't know what all your sites are about but even on the top sites it seems they weed the pages. For example I ran a search for an electronics product from 2000, only 8 years ago. You can barely find traces of it in the sites I searched via Google. Now do the same search from a product from today, say the iphone. There is probably a billion pages on that. Now I am not saying they are doing things wrong, but with the millions of pages added every day to the internet they have to delete or else run out of space perhaps. I just wish they had the ability to search the archives easily for the topics or products that are "old". Right now you can do that with Google news but not Google search. Anyway my point is I think Google looks at a site and compares all the content, then keeps some of the most recent content in the results including the higher PR stuff and puts the older stuff in supplemental. That is only a guess but seems to be what is happening. Since the older stuff I looked for was probably dropped into the deepest parts of these sites I couldn't find it with Google anymore. Maybe though this is the way the internet search will be, you use if for todays content only. If they had to archive all our sites I don't think it is possible, not with all the pages being added
You should read Google's latest patent. It talked directly about lowering the rankings of pages that seemed to get a bunch of links and then died off completely.
They're for sure doing something. I dropped like a rock on HUGE terms where ive held at least the #5 spot or better. The drops only seemed to last a day and now im back. I hate when google plays with my emotions.
Take into consideration that a product from 2002 is initaially mentioned on less web sites than todays products as the number of web sites has expoded in the last 6 years,