I noticed that I often see the same ads on my site. Meaning the same links will come up pretty often. (not at once) I also noticed that I was getting multiple backlinks from the same sites via the coop. So I went looking. I noticed that some sites have my ads - and show them often, other sites dont have my ads and dont seem to show them. Im guessing that the ad_network_ads.txt file gets full and ads kinda get locked out if they are not already in there. Maybe finding a way to clear this file from time to time would resolve this?
The cache file automatically rotates ads. Currently every 15 minutes it picks up a new ad, and drops the oldest.