I read someplace a week or so ago that they felt Google was using some form of a switch method. They described it as where a site was supposed to rank in say position 4, if Google imposed a penalty for some reason, Google would instead penalize the site maybe 10 slots and instead Google would fill slot 4 with a result from either Ebay or Amazon. The more I look at the results this does seem like a somewhat accurate description. The real question is what is the cause of the penalty. Look at your results and see if when you see Ebay or Amazon if exactly 10 to 15 positions below is a site that you think should be in that position Ebay or Amazon is in.