[Rob Doe]

Clean and tighten grounds and cable connections, if you have a safety fuse replace it and, if still failing, next bypass it, check the key switch by jiggling it up, down, side to side and that it has detentes at off and on. It should not shut down if not fully in the off detent. Tighten all connections behind the panel and in the terminal box, also at the coil.

Push the coil wire up at the coil and down at the distributor. You've had them apart.

If you have the the spark tester that Nurex sells, it can show you a misfire in the secondary circuit.

Grab the upper distributor plate by the spark lever and try to move it fore and aft etc. but in the plane parallel with the engine head. The center hole may be worn allowing the points to move around. Look at the points rub block / cam follower and make certain that it is not too worn allowing the occasional spark to jump to the cam.

Check that the condenser lower plate wires at the side of the upper plate are not sticking up too high and allowing a spark from the underside of the rotor to the connections. This would likely be consistent on #3 cylinder.

Look closely at the contact surfaces of the points. If they are pitted, burned or have high spots, suspect a condenser.

There are some voltage tests in chapter 2, pages ~30-32?? of the blue, Les Andrews, "Troubleshooting and Diagnosis" book that will identify faulty, high resistance connections, both power side and grounds.

That's all I can think of quickly.