'94 Mercedes S420/158K miles; engine runs great unless car sits unused for 5 days+. Then, sluggish initial acceleration, stalls(with rotten egg odor) until RPM hits 1500+. After 20-30 minutes, seems to clear up, then OK. Catalytic converter likely?
It could be a lot of things, but the cat converter wouldn’t be on my list. My first guess is moisture in some part of the ignition system. I would replace the distributor cap, rotor button and plug wires if it has them. Maybe even the ignition coil.
I’m gonna guess that your fuel line is draining. The next time the car sits, try turning the key to “on” for four +/- seconds to pressurize the fuel line before turning it to “start”. Post back with the results.