I dont know if you are having the same problem I was but I JUST took in my 91 300CE to the shop because it had that problem. They said that what was causing it was the temperature sensor, which was broken in my car. Since it controlled the air/gas mixture, the car had no way of determining how much of either to distribute. That may be completely wrong but that is the description I got from my dad when he explained it to me, I hope that helps.

Leighton