The ignition system is what fires the spark plug, correct?
t sounds like one of the circuits, components, in the ignition system has a component that is not working below 12.5 V twocents
I would look closely at all the grounds, make sure they are really good wrench, and then start by replacing the ignition ballast resistor first, the coil next and then the ECU shruggy
Can you have someone help you by sitting in the car and have them do the starting while you use a voltmeter to check the voltage to both sides of the ballast resistor with the starter spinning the motor over and then check the voltage to the coil and last to the ECU scope wrench
Murphy loves messing with us hot rodders rant shruggy

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)