Sputters and backfires after fifteen or twenty miles

If it is backfiring out the intake , than look for lean conditions, crossfiring, valve failures, etc. If it is firing out the exhaust then look for valve failures, volumetric efficiency (poor compression causing partial burns and fuel in exhaust), back pressure, crossfiring, etc. At Street Tech Auto Care , we could diagnose it for you and then you could fix yourself or have us do it. Either way we would be honored to help. If you post more testing results I will reply with solutiions till you get it fixed or please do call or come by and we will help anyway we can.