If wires, distributor cap, coil and spark plugs are fine and if it is not a fuel problem, then...

Could be the crank position sensor, for certain positions of crank shaft, the sensor may not output anything, i.e it is open circuit.

Does this car have an alarm system like the ML320. If it does, try another key to start the car. On my brother's ML320, the car would start and quit immediately and guess what was the problem. It was the ignition key, as soon as the car came on, the key was checked and if it was worn out in some wierd way, the car thinks that someone is trying to steal it and will shut down. We used a different key and the car started and ran like a champ.

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Saumil S. Patel