johng
+1y
This is my first post but maybe I can help?
First thing is take a piece of paper and measure the voltage with a dc meter at different points when its (cold not running) and (cold and running) (starting even is different on voltage resistor I think is bypassed for a moment). Example each side of that resistor and at the coil even if its the same wire that goes to the resistor. Also notate where your ground for each test, example if checking the coil + voltage measure you (-ground) voltage at the coils ground. If you measure at the batt for your ground or the frame it may be better and give you a false reading.
After all those notes look at your notes or points and do the same test when its hot and failing. If they are all the same you have a bad ignition module in the distributor (if it has one) or pickup coil in the distributor (if it also has one). If it has points change out condensor and points.
You can pull the distributor and ground the shell to the block and spin it instead of cranking the engine and it will spark. But make sure you note your timing mark or rotor position. Put it at top dead center (TDC) on compression stroke before you pull it for easy realigning.
Also you can spin distributor without cap and coil will spark 4 times faster. put a plug on the coil wire so it acts like a one cylinder engine.
I know a lot of random thoughts but if you sift through my thoughts maybe it will give you a new direction to troubleshoot.
Also use freeze spray on pickup or ignition module (if its not points) when failing and spark should come back if its really electronic failure.
Hope this helps