Posting for a friend. Basically the problem is comparable to a bad carburetor (though bmw's don't have them). You can't keep the car idle with out gassing it, when you hold the gas with a steady foot the rpms will slowley drop, and if you let it idle the car will just die. Gassing it too much at idle will choke out the car so you need to pump the gas peddle too get those rpms up and once there up hold you foot down, but like I said earlier the rpms will slowley drop. Any ideas? Vacuume leak, maybe fuel pump? Was running fine until one day he was stuck stranded on the road. 
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