Q: Car shuts off sporadically

My car has had an intermittent problem with shutting off completely at certain situations. This occurs both while braking to a stop, like at the traffic light, as well as while accelerating from a stop. The car would shut off in these conditions and always at lower speeds. I would then have to put it in park and start it up again. There are no problems with starting the car. It starts immediately. The dealers cannot recreate the problem so they cannot solve it. No error codes came up. The repairs done so far include the alternator replacement and throttle valve control unit carbon buildup cleaning. What could be causing this?