I have a 84 mustang gt 5.0 carb with all orginal electrical ignition. sometimes it runs fine but getting worst about shuting off. when it does this it starts and shuts off as soon as you let go of the key switch. If I hold it in the start position it will run but grinding starter, I have changed ignition switch 3 times new coil and 2 different dura spark ignition boxes. its not fuel related it electical getting gas. 12 volts on pos side of coil when switch is in run position. Ran a jumper wire from batt to coil and used a screwdriver on starter relay to bypass ignition switch but would not start. Im at my ends dont know what esle to try please help......uploadiing wiring diagram if someone needs to see it. The 5.0 wiring is in the little picture for the coil