1999 Pontiac Grand Prix having a small problem

Yes I just got my car out of the shop I had a motor put in now the motor was bought by us fidelis and I drove my car and it keep dying around turns, stop lights & signs, yeild signs lol and the next day it wouldnt start turns out it was the cranck shaft sensor. Replaced sensor now it still dies around turns stop signs yield signs, my service engin code last read before motor was replaced was mass air flow sensor. Would cleaning it fix my problem or should I go replace it? And in the morings or when it sits for a couple of hours it has a hard tim starting the rpm's jump and then it dies but the rpm's go up and down when idel untell the car dies, but the hard to start could be the air flow sensor or a bad battery?