my bmw stall when i brake in traffic?what could be the problem?

probable vacuum leak - a smoke test would locate it. You should take this very sophisticated, technologically advanced piece of engineering beauty to a trained BMW Specialist (a specialist is someone who only works on one make of vehicle). There are many qualified independent, as well as dealer choices out there. Remember, there is no such thing as a Euro Specialist, or a Japanese Specialist, or a Domestic Specialist, or even a German Auto Specialist - the term would be an oxymoron. A German Auto Specialist would have to be proficient in seven different makes - this is not possible anymore "Jack of all trades, master of none". You can look up an excellent BMW shop by going to the BIMRS web directory (non-profit) at bimrs.org