In reading the posts I didn't see what type of ignition you have? Does it have points and condensor? Is it solid state electronic?
If it has points and condensor, it may be the condensor is bad.
Check the battery area and make sure that the battery hold down bracket, is not touching any of the battery terminals, in effect grounding the battery intermittenly. Check ALL the grounds. Check the voltage at the battery and ALL the other HOT wires.

This thing as you describe was also happening on a John Deere farm tractor, owned by a Doctor, friend of mine. It took a couple of weekends tracking and checking the little things, until we discovered the the guilty party. It was a bad condensor, along with the other things I have mentioned. ALL of them helped in keeping the tractor from starting and running, and they were intermittant, so the problem kept changing.

Good Luck!

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SONNY was right...Nobody Cares.
Calogero, from the movie, "A BRONX TALE."