3.8 Sable temp gauge shoots up

Alright I replaced both head gaskets about 6 months ago - had head planed new gaskets, etc. Car started right up, white smoke disappeared. Car was still running hot initially until I replaced the coolant cap. (Surprising what a 5 dollar radiator cap will cause). Car is running well, but when I turn on my airconditioner the temp shoots up about 2 letters (from m to to the n in N-O-R-M-A-L). It happens in about 2 seconds - too fast for the temperature to have actually changed that fast and it always happens when I turn the a/c on.
Occassionally it runs high normal, but sometimes without the a/c on it will rapidly rise a couple of letters and randomly drop back just as quickly. All these temp changes appear way too quick for the coolant to actually be changing that quick.
What effect would the A/C have on the temp. Could the A/C be causing some kind of interference with temp sending unit? Could the temp sending unit be bad? Could it be the upper intake manifold leaking? Guys I'm lost. Little help....