I had the EXACT same problem. It would occur only on warmer days, and would be noticably worse on humid days. Guess what? It is throttle hang. Correct me if I'm wrong here; pumping air into the engine will lean out the fuel/air mixture which will increase the rpm's. The throttle hang/rev increase is the result of the engine pumping to much air as part of an effort to control emissions. Putting the copper plug into the air line acts as a restrictor and doesn't allow the engine to pull to much air.
I could be all, doesn't matter. What matters is that the problem should be fixed with the plug. I got sick of my car revving up to 4500 rpm's, and finally went and broke the bank on some $0.25 hardware.
