Idle - slight RPM fluctuation 420 SEL

My '91 420SEL (151K miles) has an ever so slight, regular variation in the RPM's at idle - it is barely noticeable, I can hear it and see it in the oil pressure gauge (once engine is hot), but it is almost unnoticeable at the tach. BUT I CAN HEAR IT! The idle speed seems to fluctuate EVER so slightly, almost rhythmically, every 1.5 seconds? The overall speed on the tach seems to be right on: 500 RPMs in gear, and aprox 700 in neutral.

Background - I recently had a miss/stumble that was cured by various ignition tune-ups: replaced plug wires, distributor cap, rotor, spark plugs, as well as O2 Sensor. I also had the Motorvac service on the injectors.

Is this slight fluctuation just me being over sensitive???? Is this normal, and I just didn't notice it with the other problems on my car?

Any suggestions/insight/advice would be appreciated

Ljones