I have spent half the day going through a site that was hit with 50% reduction in traffic.
I found about 20 "thin" articles and replaced them all with fresh new original articles all over 500 words. After looking at these articles they are pretty much worthless and I could see why they would be an issue.
I also noindexed the contact page, and any other page that were not needed.
There is nothing else I can see to do at this point.
I will report back if there are any changes.
Will give this about 10 days, next step is to reduce adsense units, there are only 2 on each page but will go to 1 if this does not help.