I would check the carb. I would replace the plug wires, time, replace the plugs. Look at the plugs for consistancy. Spark plugs are a great thermometer.
A point an old racer showed me, do not run the plug wires parallel. They can induce misfire under certain conditions by induction. I always cross mine at least twice from spark plug to distributor. I would look for a weak lifter.
It sounds like it is one of those things.