I would check for any type of leak...I remember having a similar incident - and the rubber boot that comes from the mass air meter to the engine was very loose (the clamp) almost coming off - the day before the incident, I had a SES light, and the next day the X just stalled as I was about to make a left turn (me at the front of the line). I waited thinking on what to do - then I restarted the X and it was normal - went to work and on the afternoon that I came back home, I checked and that rubber boot was loose. Check this:

http://www.realoem.com/bmw/showparts...12&hg=13&fg=15

I hope this helps