Gearbox problem?
 

Approximately 2 years ago I had my automatic gearbox on the E53 rebuilt as it had failed. Over the past few months I've noticed occasionally when I put the car into reverse it will want to shoot backwards if my foot isn't firmly on the brake, almost as if the clutch is not engaged.
Other times I can put it in reverse and gently press the gas pedal and it will just stall or the engine will stutter slightly (especially when cold) and then it will shoot backwards.
I don't recall it ever doing this before the rebuild, so I'm curious if this is likely to become an (expensive) problem to sort out.