Found this after years of chasing this ghost. Started out after hours of running the engine would start losing power and finally stop and would not restart until it cooled off. Came home on one engine quite often.

i was able to duplicate the problem while at dock without taking it out and risking getting towed back home.

What I found is the clip down distributor cap was losing continuity between the center post and the spring loaded carbon pencil inside the cap that makes contact with the rotor'

I found this out by using two spark plug testers.

One from the coil wire to the center post in the cap and the other one attached to a spark plug. I started the engine up and watched the two flashing testers. After a 1/2 hour, I head a slight miss and observed the tester at the top of the distributor was flashing steady but the tester on the plug was not. The engine finally stalled. I pulled the cap and checked for continuity between the top center post and the end of the carbon pencil. There was NONE.
My theory is the spring that holds the carbon pencil to the top of the rotor has failed when it warms up.

Try this simple test before you start changing parts. Fuel filters, coils, rebuild carbs. replace ignition modules, wires plugs, etc.

Hope this save someone the aggravation I went thru.
Bill