BMW Repair: BMW 318is is running poor, throttle position sensor, bmw 318is

Question
I have 1993 BMW 318is, it has only 98, 000 km on it. My problem is that my car doesn't run properly all the time, sometimes it works just fine , sometimes it is doesn't. 'Check engine indicator comes time to time but doesn't stay, it goes away when I gas pedal is released.  Sometimes when I step on gas pedal nothing happen, car doesn't go accelerate, then it would pick up and run high, until I drop pedal again. Somethimes car would run rough, rpm indicator would jump up and down, and car sounds like a motorcycle engine  Mechanic doesn't know what to do with this car, he replaced oxygen sensor and thermostat.  I think it has to do with air flow, because engine gets floated, spark plugs  are black. Any idea what is happening. Thanks

Answer
george,
Would it be safe to assume you are not in the USA?  If this is the case, I may not have enough information to properly diagnose the problem.  I'm not sure what your trying to say when you said:  "car doesn't go accelerate, then it would pick up and run high, until I drop pedal again.".  
check the throttle position sensor, airflw sensor and the coolent temperature sensor.  The best way to diagnose the problem is to put the car on a 5 gas analizer and an ignition scope.  The excessively rich mixture will cause the engine to stumble on accelation, other causes could be the ignition system but as I said its hard to tell whats wrong without looking at the spark pattern.  Your car has a diagnostic port; if you plug a computer into it, the car will give you a history of all the engine lights and what has been causeing them, go get a new mechanic (the thermostat has NOTHING to do with your problem and the O2 sensor does not have the ability to cause your car to run as poorly as it does; my 02 sensor is unplugged and the car runs excellent).  When you find a new mechanic, tell him to drive the car around for a while untill it starts to run poorly, at which point he should plug in the computer and check to see what the engines' computer has to say about the poor running.  Or, you could buy the reader yourself.  When you figure out what is causeing the problem, I can help you more.
-cheers
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