Check the battery connection, coil, distributor, ignition switch.
Sounds like a wire / connection is getting hot in normal operation and then not flowing enough current.
Why don't you isolate things when the problem shows itself - to eliminate that component.
Have some spare parts available. Coil, battery, jump leads, heavy gauge wire.
When the problem shows itself try jump starting the car directly to the starter motor from a spare battery. Try jumping between the starter solenoid and the starter motor - do you know how to do that? (be careful things will be hot!!)
Jump the wires from the ignition switch - do you know how to do that?
Swap over to a spare coil. Swap out the distributor cap for a spare one.
Swap over the coil leads for a spare set.
BUT - MY BEST GUESS IS THAT….. it will be a bad earth somewhere. Either from the engine block, the battery to body connection, or possibly the ignition switch connectors, the multi plug under the steering column cowl, or something like that.
CHECK your earths first, check the mulitplug in the steering column, check the battery and its connections, then start isolating / swapping things out / over - until you track down the problem.
I hope that helps.
Let us know how you get on…..