06 530i BMW replace starter/ running problems

Hi, I have a 2006 530I bmw, I just had the starter replaced, however when the car starts it shakes alittle and the needle struggles to stay at a steady past. When I step on the gas a few times the needle catches it bearings and stays steady. I can then drive the car and it drives smoothly until I have to step on the brakes (example, stop at a red light) the needle becomes unsteady again moving up and down as if it cannot function without my foot being on the gas, it even cutsoff if I'm on the brakes too long. Can you tell me what the problem is? Or do you have any ideas what the problem my be?