netmeg - 2:00 pm on May 11, 2013 (gmt 0)
I would look at origins of traffic, and your placements.
I agree netmeg, something is definitely wrong. Unfortunately, I have no idea what it is.
If you are satisfied that your traffic is okay, then look at your placements. Make sure each one has its own custom channel, and see if there are any patterns to the take-backs - if you find it's usually one ad unit, maybe you need to make some changes there; make it more or less blended, or move it over a few pixels. But having that much lopped off every day is a signal something is really wrong.