We changed out a 1.8 V6 and put in a 2.5 V6, right off the bat, the distributor went bad, we put a external coil pack on. the car runs fine until you turn on the head lights, it spits and sputters and we have no idea's whats causing it,,,,,,,, any answers would be helpful,,, oh by the way,,,,,no problem smoking a Mustang GT from a dead start. I think we have a 2300 pound rocket on wheels now.

Check out the wiring for good connections. This includes the ones from the battery posts to the terminals. Also make sure that everything has its own + and - wiring. Don't splice into any wires.