I'm sorry I can't help with the gearbox, but it definitely sounds faulty. I would get it looked at.
If you find the car rolls back when you are facing up a hill, why not try pulling the handbrake on? If it is the auto handbrake with a switch, it will release as the engine starts tries to move the car. If it is the traditional lever type, then as the front of the car starts to rise, release the handbrake.
Good luck!