96 grand am se

I have a '96 grand am se 2.4L. This hesitates when I take off. It sometimes backfires and sometimes even dies. I changed fuel filter, no difference. I changed spark plugs and the car ran great for about 40 miles. Later the alternator went out, and I put a new one in. Once again the car ran great for about 40 miles. I realize there could be a lot of different things causing this problem. Does anyone know exactly or maybe even acouple of things it could be? Thank you. Darrel.