[JM 35 Sedan]

Bypassing the ignition switch, as mentioned above, would be something to try. Contacts in the switch could be a high resistance connection that heats up and becomes a worse connection as it gets hotter. This also applies to other connections in the ignition circuit. A stranded wire with most or possibly all the strands broken, but still touching making contact, will eventually get hot enough to cause an open circuit. Sometimes a terminal poorly crimped to a wire will cause a high resistance connection that gets worse with heat.
A coil with shorted or burned open windings may seemingly check okay with an ohmmeter but fail to work properly as the windings heat up. Sometimes while trouble shooting problems like this we inadvertently replace defective components with other components that are also defective.

I had a similar situation with my '35 fordor last month and the problem turned out to be a poor/intermittent solder connection of the grounding strap to the back of the condenser. Repaired that and completely went through the distributor, cleaning, oiling and resetting everything. It has been running great for the last 200 miles.