I have exactly the same problem with my W123 280E, and several independent mechanics have failed to find the source of the problem.
But I recently took the car for a service to an M-B dealer, and they traced the problem to a faulty fuel distributor.
I don't know if your car has K-Jet, but in my case, petrol is leaking past a worn plunger inside the fuel distributor. It only causes a problem when the fuel and airflow are low, hence the car drives fine unless it's idling. At idle, it drops to four or five cylinders.
I haven't yet had it fixed. It's an irritant, but in a way, it's also a fuel-saving device because not enough petrol is getting through at idle!