John,
You have done a whole lot of stuff so the field of problems has been narrowed. It would seem to me it's either fire or fuel.
As the engine warms up, the coil warms with it and this will increase electrical resistance. If there is a internal short or turn to turn short, this increased resistance can cause the coil output to fall below the threshold it takes to properly fire the plugs. So a bad coil is a possibility.
I would try squirting some fuel into the carburetor throat next time it refused to start. If it fires, then lack of fuel seems like it would be the culprit. Most often, a warm engine, after it is shut off can cause the carburetor to heat and the fuel to percolate out, dumping into the intake and flooding the engine.
You would see some evidence of black smoke out the back (maybe), and it would cough and sputter as it came to life. This would be a sure sign of flooding. Do you still have the little plastic spacer between your intake and carburetor?
Taking it one step further, a weak spark in conjunction with a rich air/fuel mixture would create a 'hot' hard to start situation.
I would look at all these things, but I would also make 100% sure my point gap was perfect. Keep us posted.
Stuart
