Engine Vibration

I have a '93 300E 2.8 with 140K miles on it.  There is noticeable vibration when car is idling waiting for light change and in D gear.  The vibration will be less if I shift to N and will be a bit worse if I turn on air-condition.  But there is not noticeable vibration when driving the car.   Recently if I start the engine and drive less than 3 min. and let car sit for a while or over night, sometime it will develop serious vibration when start the engine again.  The serious engine vibration will went away around 30 seconds later.  Last week the same situation happend again but this time the engine stopped and can not start again.  Next day I was able start the engine but press the gas paddle to give more gas to engine.  The engine started but exhaust smell very bad for a while.   What is the possible problem(s)?