It sounds like you have a electrical "high joint" AKA high resistance open, when one part of the ignition feed circuit gets hot and expands and stops conducting at low RPM due to low voltage from the alternator. When you go to start it the 12V direct to the coil bypasses the ballast resistor and makes the motor start, when you rev it up the increased current jumps the high joint and allows th emotor to run until the RPM go down low enough to stop the current flow again.
1969 was a very bad year on Mopar cars electrical systems
It will take some time and effort to find it and fix it but you can do it, don't quit.
I would start at the coil and work back through those two circuits, both the start and run feeds to the ballast resistor and then into the coil, remove all the slip fit connectors and clean and tighten them and then retests them, one at a time
The bulkhead connector can be a real challenge to clean ad tighten but it can be done by removing them one at a time from both sides of the bulkhead connector and look at them real hard and see if they slide off and on easy or hard. Clean and hard is better
Are you still using the original mechanical voltage regulator and single field alternator?
If so check the voltage that it is running on when cold and again right before it shuts off when idling warm or hot..
I'm thinking it may be dying due to the volts dropping below the run voltage that ECU has to have too idle when warmed up