Let you in on a little pet theory of mine….
Several of the PPCs mentioned act as a trigger for Gator, WhenU and the larger Scumware operators. They monitor the listings and place their pops on the sites they see assuming that the traffic will be there and they can pick off visitors for free…
I noticed when my listings come down my conversions actually go way UP.
I am doing further tests to determine the extent of the problem and which PPCs are targeted.