I have a similar issue
I am having a similar issue with my 2013 X-3. Twice I have slowed for a stop light, only to have reduced braking power and then a surge of acceleration. The engineer for BMW of North America has inspected the car and they tell me they cannot find anything wrong. I totally understand what knoxvolgirl is saying. I can press as hard as I can on the brake and I can tell that it is just not going to completely stop and then I have a surge. I hit a car stopped at a red light one day because of this and the other time I sailed into an intersection...luckily noone was coming and I was able to turn into the street and stop.