I would first concentrate on the ignition >
As ignition components warm up if a weak area is evident it will brake down often .
Coil / Plug and Spark Wires / Resistors / Points and " Condensers as example
First thing I would check is how much spark it coming from the Coil wire to Ground as well as the Plug wires > Check it cold < Then same check when its Hot or warmed up
( can do this just cranking with the key on ) < Trigger Points / Fuel Pump Relay >