chrisv1963 - 6:50 am on May 25, 2013 (gmt 0)
Same drop in Google Traffic here, starting on May 7th/8th. This is for my good website with a lot of original content and updated almost daily.
Weird: At about the same time pages from an old rather crappy website I own (Not updated in about 8 years!) started showing up again on the first page of the serps.
This must be some "quality penalty". If the quality of your content is too good, then it is clear that you are trying to get good rankings => seo spam => penalty! :-)