Strange Idle Problem
 

The idle on my 81 380sl drops and then immediately rises every 15 seconds or so when the the car is in P or N when the heater is off. However, if I turn the heater on or put the car in gear the idle stays steady.

I replaced the idle control unit with a rebuilt one twice now and the idle control valve seems to be fine. So, I'm assuming it has something to do with the fact that the AC compressor comes on when the heater does, and the effect this has on the vacuum advance and/or changeover valve. But I'm not sure how to correct the problem or even diagnose it. So, I was hoping someone here could tell me what to look for.

Thanks very much in advance.