tedster - 2:06 pm on Aug 13, 2012 (gmt 0)

Site drops 7 pages in SERPs overnight and then gradually recovers over next 3 weeks WITHOUT DOING ANY CORRECTIVE SEO Levels off for a week, then the site drops 3 pages in SERPs overnight and then gradually recovers over next 3 weeks again WITHOUT DOING ANY CORRECTIVE SEO

This sounds like the "sine wave" phenomenon others reported lasted year and the year before. Can't say we've ever came to a deep understanding of it here, but know that you are not alone.

like they have been playing dice

Yes, it does feel like some kind of statistical study is going on - especially below page 1 or sometimes even ON page 1 for some lower volume searches.