i have a similar problem with our '87 300TDT. the engine doesn't always stall, but the idle drops quite a bit with the compressor engaged. this is my wife's daily car and yesterday she had a problem with the car stalling in heavy traffic. once she managed to get back home we looked at it and realized it was the ACC that was dropping the idle, economy mode was fine and other modes dropped it.

the compressor is turning, so it isn't technically seized. is it possible for it to be "partially seized" so as to just cause a heavy drag on the engine? there are no odd noises or behavior to suggest the compressor is bad except the stalling/low idle issue.