My 3.0 D has started to surge/accelerate after selecting reverse. If you brake to stop it the engine stalls. When you restart, it can do it again but usually it only happens once each time. It doesn't happen every time but is worse when engine is cold and particularly bad when reversing up a steepish slope.

The gearbox was rebuilt 60000 kms ago with an upgraded 'Cooper' kit and has been regularly serviced since.

I'm unsure whether this is a gearbox or an engine management problem (or something else?).

Any advice would be greatly appreciated!