I'm not sure if i am on the right track becuase what model is your car? A similar problem on a 124 would or could be the Kilma relay(just behind the battery) becuase that relay reads speeds from the engine(tach meter) and then from the ac and checks if they are the same, if not the ac compressor cuts out. So either the speed signal from the engine is not 100%(not sure why) or the Kilma could be faulty or loose. This isn't concrete advise but it might help.