For some time now, the homepage of my site has been bouncing in and out of the SERPS for our main keyword. It hovers around pages 15-17, then it will drop out completely a few days later. I'm assuming there is some sort of penalty applied to it to a) rank that low and b) drop out entirely, but I'm not sure where to start looking. Is this type of behavior typically indicative of something? Over optimization? I don't do any link building. The onpage content does change a little each day, but things like keyword density shouldn't be fluctuating that much because the content is the same type of content every day. The homepage does come up with sitelinks if I search for our brand, but it's our main keyword that is showing this other behavior. Other pertinent info: domain is about 12 years old and has always been well favored in Google. We did get hit by Panda, but this fluctuation was going on before that (I'm just now trying to figure out why).