Engine surging with A/C on, 99 GT

When I'm driving, with the A/C on, the engine surges big time, it sucks to drive with the a/c on, but I live in FL and need it. A/C blows cold air, but when the surge occurs, I hear a vent/duct slamming shut under the dash. Also, when at idle, the a/c cyles, and the idle jumps up and down every min. or so. Did a search, didn't come up with anything solid. Saw it might be IAC or an A/C pressure sensor.... Anybody seen this before? I could just start shot-gunning parts, but hate to replace an IAC or anything else for that matter, when I don't have to.
 
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because you have to relearn idle strategy again. disconnect the battery, and idle the car with all accessoriess off and the a/c in the off position.

when you reconnected the battery, you might of had the a/c on while relearning the idle strategy. started it up with the a/c on, and learned an idle strategy that already has a load on it. thus dropping the rpms even lower when the load is off.
 
The car does it up thru the rev range, while cruising down the road @ 2000 - 2500 rpms, or even higher, it will constantly cycle the a/c(even though it's blowing cold) and when this happens, the car surges, with a noticeable loss in power everytime it does this.
 
Reflashed to stock, still does it. Put the tune in before I really had a chance to drive the car, I don't remember if it did it before I did the tune, only drove it home from the seller's house with the stock tune, and parked it.
 
Well, I just went out and troubleshot it. Disconnected the A/C press switch and jumped the connector, now it works great. So now it looks like either a borderline A/C charge, or press switch is bad. A/C still blows cold though.
 
Usually the cause of this problem is low freon. Recharging it should fix the problem. A low charge causes the compressor to cycle on and off too quickly which lags the engine.