I've thought about the A/C Compressor too. However, I would tend to think that if the compressor was bad, the AC performance would be effected, and it works great. Also, this only happens in Drive and I can't imagine the gear selection would effect the A/C that much.
Altenator? I guess its possible, I haven't had it tested but I know the battery is fine.
Do a visual inspection first, for any cracked connections or lines. Two places to pay close attention are the t under the supercharger snout from the LIM, and the capped off vacuum fitting on top of the supercharger. Spray throttle body cleaner around connections and gaskets. See if the idle changes any.