[Bruce Lancaster]

Could well be a marginal coil or condenser checking out when hot.
Engine starting happily later suggests that carbs are still full and able to function, but here's a simple test: With engine in no-start mode, put a small splash of gas or a shot of starting fluid down the throats. As you crank, engine shoulf arrive at a burnable mixture and try to start if fuel supply is the problem...I'm pretty sure your problem will prove to be ignition parts on their way out.
PS...Mallory condensers are as marginal as their points in my experience...the condensers hav very short electrical lifespan AND they have grounding problems due to a shellac-looking coating on the can.