I have a 91 300E as well. 186K. Your Idle while in Neutral or park should be about 750 RPM. When in drive with AC on, about 550-600. If it feels like it is going to stall, could be ignition system. I replaced O2 Sensor...improved idle. Replaced coil, and rotor cap. Not much improvement. Replaced water temp sensor and improved a little more. Now, I have been having cold start issues lately where it seems like it is flooded. I have read that it could be the EHA Valve that pressurizes and warms the gas before going to engine. It starts up fine when warm outside or engine is still warm. However, I'm going to put new plug wires on first (76.00 from *********.com) because I heard and noticed "ticking" noises from the engine with hood open and this will cause lack of power when under load and idle problems while stopped at light. Plug wires are usually the last things people change. If I were you, I would replce the O2 Sensor first....should notice a big improvement because it sends the adjustment signal for air/fuel to computer based on exhaust output. Let me know if you need help. I bought the 95 Ford Mustang O2 Sensor (Bosch)and just spliced the wires in. Saved me $150.00. Same sensor as Mercedes Bosh replacment without the wire harness connector. Last thing after all that is adjusting the lambda tower (little hole on top of the Air Filter housing) but this could get you into a no start situation and then you're screwed. Takes tiny adjustment turns with 3mm allen wrench.

Steve