On one of our domains I'm seeing some strange "periodic Google filter" (penalty).
It started March 22nd when Google traffic was going down by 75% for two days. Just one directory was affected, the rest of the site stayed as the days before.
After the two days everything recovered to nearly normal (back to 90% of Google traffic). One week later the same story - the traffic was going down for two days by 75%. And it is going on now for the 4th week. The only thing which has changed is that the drop it's now on Wednesday and Thursday while it was on Thursday and Friday at the beginning.
The reason for the decrease of Google traffic is some kind of filter (penalty). On the first day the keywords are gone. On the second day the reappear but are not ranked well (page 2 or 3). On the third day everything goes back to nearly normal.
It seems that the effect is just for one and two word searches while long tail listing are not affected. It seems that the pages are filtered out because "site:example.com keyword" is still showing the original page on the top while a search just for "keyword" brings up a different page which is not part of the affected directory (but this page isn't ranked very well).
For me it seems that an filter is automatically applied to a directory one day and automatically lifted the other day - strange. Has someone else observed something similar?