Q: Fixed crank shaft position sensor and engine light still on 2001 BMW 325i

Okay so My car had been stalling to start and the check engine line came on, so natrually I took it to my dad's mechanic. He then put the scanner in my car and said it was the crank shaft position sensor, so I bought it and he had installed it and that day the light had gone away, but thecar was still having trouble starting so he assumed it was the spark plugs that needed changing, so the next day i went to buy the spark plugs and the light came on again and i called him and he said hed take a look at it when he changed the spark plugs, so when he came, he changed the spark plugs and put the scanner in again and it said that the crank shaft position sensor was acting up again, so my question is What could be wrong with my car that it's not accepting this new crank shaft? because he reccomends getting the crank shaft straight from the bmw dealer instead of another brand but I don't want to keep paying for a new crankshaft and paying the labor fees if the car will keep acting up

My car has 149000 miles.
My car has an automatic transmission.