Stalls when shifting into reverse

What is your question?

Hi there, My Focus has started to stall when I shift it into reverse - and only reverse. It used to work intermittently but now it is almost all the time. It will only work if the car sits for 4-5+ hours will it start to work again. I recently had a new fuel pump installed because the engine was stalling randomly. Not sure if the two are related or if I got some bum advice/repairs. The car is an automatic.