My guess from 1800 miles away is condenser or coil heating up and quitting. When it cools down it starts working. Easy way to tell, next time it quits, look down the carb and work the throttle. See if the accelerator pump squirts gas. if it does you are not out of gas. Pull a plug wire, stick a spare plug on it, turn the motor over and see if you have spark. If no spark, try replacing the condenser. If you have gas and spark and compression and it still quits it must be a voodoo curse.