Issues with reverse
 

I have a 2003 325i that when in reverse, primarily while braking tries to kill. Have taken it in and was told it is not a tranny issue. Was told that it is registering too rich of a fuel mixture so is cutting out fuel which is causing the car to attempt to stall, if brake pedal is released before the stall fuel mixture is adjusted causing car to shoot backwards as fuel is returned. Shop has done major systems tests and not turned up anything, no codes are tripped and that I should continue to drive to see if problem gets worse which would hopefully make it easier to diagnose. Any ideas???