AdTrafficQuality - 4:44 am on Sep 20, 2012 (gmt 0)
Hi jost -
I would take advantage of some of the tools available through your AdSense account to try and isolate where you feel these issues are coming from. Maybe you have already, but I'm unsure from the information provided in this thread.
We have provided an article at [support.google.com...] that describes ways to segment traffic. This might shed some light as to where users are coming from and which ad units are affected. Hopefully this can help you isolate any issues and take action.
Hope that helps!