skagitsubaru

First time poster here, have gotten some good advice from this forum. I have a 1993 Legacy wagon with the original 2.2L ej22 motor, which has developed a perplexing problem. When I engage the a/c (defroster also), the motor has a serious rpm drop when the a/c disengages. This problem developed when I started to have difficulties with the a/c, which led me to have the compressor changed, system swapped to the new freon stuff, and the problem continues. It doesn't matter whether I am at idle, driving steady at freeway speeds, whenever the compressor stops, the car has such a rpm drop that it feels like a stall, or even like I hit something. I started it up today and let the car idle while turning on the defroster, and watched from the front; sure enough, when the a/c stops, the engine drops rpms (like 500 rpm) and almost stops, and it takes almost 2 seconds for the engine to recover. It takes the engage portion fine, speeding up a little right before the compressor engages. Anyone else experience something like this? All the searches turned up drops when the compressor engages, not when it disengages. Almost like the computer is over compensating for the end of the power drain. Any ideas? I suspected the iac, but the fact that the engage portion of the a/c works fine causes me to question that conclusion.