2001 Ford Mustang GT backfires when starting, slow crank

I have a 2001 GT 8 cylinder. It has a 127 K miles. It backfires when starting. The gauges on the dash would go from left to right back to the left and then level off on the right. I replaced the battery and it fixed this problem. The check engine light has since went on and the car rumbles and backfires when it starts or won't start at all. The battery cranks good and the car will start from time to time. Could it be a timing issue or something electrical? Any suggestions or see the mechanic?Thanks!

I had the code checked as suggested. The code comes up Camshaft postion sensor Bank A. Is it just the sensor or should I expect other problems as well as or after replacing the sensor?