I use pop-unders. I place them only on the proxified pages not specially top or bottom cos its the same anyway. The best paying pop-under advertiser for now is Adversal. I heard from others that GI pays about $0.30-$0.50 cpm for their pop-under which is pretty good while Adversal pays $0.83 cpm for their pop-unders.
I have tried Canep Pop-unders before and I gotta say their technology is not advanced, the latest IE version 7 actually blocks their pop-unders from appearing. I am not so sure about the technology of GI pop unders but I know Adversal pop-unders have the best technology in escaping pop-up blockers however adversal pop-unders tend to crash firefox browser. But heck who cares, 100% of schools all use IE :P
As for traffic decreasing..hmm lets just say when I placed Adversal's pop-unders one fine day on just 1 proxy to test, the visitors increased by 1000 the next 2 days..weird and ironic i know