rough idle - intermittant

[pdxman]

I have an '84 300D with 155K. Since the cooler weather has arrived, I have noticed a 'rough idle' when the car has not warmed up. the 'rough idle' I am describing is not in the manner that the car runs rough though, it seems more like a hard stress is being applied to it. It seems to be related to when the heater is on and the climate control cycles on or off. I know this because when it went into one of these phases (right when I heard the compressor kick on) the car started to do its thing and as soon as I turned the heat completely off it immediately stopped. It also seems to be a little more pronounced when in drive with the brake applied vs. in park at idle.

My questions; This is the first car I've had with the climate control (which P.O. replaced once since new). With the heat on, should the compressor still cycle on/off? I hear the occassional 'clicks' when running heat.

Could this be a compressor failure where it would be seizing or something and the engine has to work really hard when compressor is engaged?

The car does some shaking when this happens but I also noticed that the engine mounts look mushy and probably due for replacement.

Thanks anyone.