I have seen this happen to a wide range of sites, some owned by me and managed by myself and some that are totally unrelated. There is only one pattern: That’s change and get dropped for three months. It’s totally absurd to penalize legitimate sites for updating their content, I mean what does Google want – a bunch of old content in it’s index?? (I know this has been the common moan for years)
Google really needs to tweak it’s algo so it realises that a site mostly the same has not been sold or switched for spam. How hard can it be? Example: Site A is still on topic and only 60% has changed, the likelihood is that it’s the same site, not some dodgy spam. These filters seriously need to be relaxed by a great deal in my opinion.
We all know that their are exceptions, if the site is trusted massively, then it’s probably not going to drop – but for most sites, they are not authority sites that have that trust box checked and thus are going to take a long absence from the SERPS.
Matt Cutts: I implore you to review this nano part of the Algo and let sites have more flexibility to update and keep themselves current.