if this is like a 1990 300e sort of model I would suggest changing the spark plugs. My 1990 300TE did exactly that and I danced all around it, thinking it must be the fuel distributor, the O2 sensor, poor oil pressure and whatever...turned out to be a new set of Bosch Copper-Plus spark plugs. Threw $25 of plugs in it, tested the result, and as my wife said, she wouldn't even drive the thing at the time (hard warm starting, poor idle, occasional stalling at stop) and came back after I told her to try it again; It's like a whole different car...been that way ever since...it was the plugs....