Are you using adsense by any chance ,if so,are all your sites showing in google webmaster tools?
I do not think it is google dance. Must be some kind of trust reduction. Must be because of the link quality.
I lost ranking on one of my sites, although i found out it was to do with my cache not working properly (w3 total cache on wordpress) it was working for browsers but as soon as i tried to access it from google or wget on my server it returned a 500 error.
I fixed the error but still i didn't get my serps back. a week later i just pinged all my old links again and bang! i returned higher than ever.
Maybe try pinging your old links?