I have an 85 190E, and somewhat similar problem, but it happens only when the engine is cold (rough idle, hesitation, etc.). On a hot engine the car runs smooth. The dealer changed spark plugs, cap/rotor, wires, fuel injectors, OVP relay, and thermostat. Still the same problem. So its definitely a fuel system problem.
Sometimes I hear a click from the bottom, driver side engine compartment, and after that the engine smoothes out. The click is quite pronounced - I can even feel it in the accelerator peddle. Could it have something to do with the air flow sensor/plunger unit? PLease help!!