[Rock Hornbuckle]

With an ohm meter, check the readings between the engine/frame, engine/coil should read open, engine/battery ground post, battery ground post/generator ground. If all the readings are the same, you don't have a grounding problem. Do these checks cold, then do the checks when the engine is hot and quits. Write down the readings so you can compare them. Remember, in the physical world, heat and resistance are directly proportional in all things (a basic law of physics). As your engine heats up so does electrical resistance, which can cause less current flow and voltage changes. I know this sounds crazy, but electrical resistance in your engine block and wiring changes with temperature. If you have any bad wires (thinning through age) the slightest heat rise will effect it's current carrying capabilities. The same holds true for any bad or loose or intermittant ground connections. You have used a carburetor that was known to be good, also many other components changed out, with no positive results. The only thing left is what the heat is causing. Good luck.