Same problem here
I've had my car at two different dealers about 6 times for this problem and they can't seem to figure it out since it doesn't throw any codes and occurs randomly. Only thing I've noticed is it occurs when the car is pretty warm at idle with A/C on and less than 1/2 tank gas. When the idle fluctuates between 600 and 1k and almost stalls then bounces back over 1k. The minute I turn off the A/C idle is dead stable. Very embarrassing when you have people in your car and start to happen. I gave up on the problem and BMW probably has as well!