Regarding Panda 2.1 it does seem that the most obvious factor that links the sites of mine that are affected is Adsense.
Sites on the same ip but with no adsense or a different adsense account seem unaffected.
Sites with the same adsense on a different ip are affected.
However that doesn't necessarily mean that it's my utilization of adsense that is being "penalized" (although I'm not sure what is)(subject of a different thread).
I've just bought a new domain name from someone that was at the top of page two for it's keyword. It'll be interesting to see if it plummets when I put adsense on it. ie is there an ongoing blackmark attached to my adsense code or was it applied in Panda 2.1 as a one off.
Will report back in due course in case it's of interest to anyone