Could be that the compressor is wearing out and can't make pressure. My 97 SL 320 uses a Nippondenso 10PA17VC compressor. These have a bolt on manifold that uses a molded o ring. It is possible for the o ring to leak between the high and lo side causing poor performance at low speed. ( I have not verified this is a common problem. )

Auto AC is a tricky thing, The compressor is good for only so many RPM so the pulley need to be sized for engine red line RPM.

Use too large a compressor to cover idle cooling needs and it will drag the engine down at idle, cause a slight surge at road speed and it will rapid cycle ( clutch wear ).

Do any of your cars have a AC idle kicker? Some cars ( non make specific ) use a solenoid to boost idle to prevent stalling and provide some enhanced cooling. The added throttle covered AC drag + 100 RPM. ( not enough to make the car surge against the brakes when stopped )

Air in the system could cause this problem too.