I would start by checking the usual suspects - points, coil, wiring, plugs and even the ignition switch itself (though this last item is much less likely). While I always like to leave the carbs for last, on a 73, the HIF4 carbs have a thermal auto-setting for the jet heights. The fact that it works when cooler could be that the setting is off. The bi-metal unit (Victoria British P/N 3-8139) works like an thermostat and can eventually wear out, but I would first check the other more obvious things before fiddling with that especially since you say the engine catches at the end.
I hope that helps you. Good luck.
Art
(copied from old message board at the request of Art Isaacs)