You have a bad connectio, or corrosion, in the circuit feeding 12V to the MSD box or between the box and coil or maybe the mag pickup connections Something is heating up and breaking the connection stopping the power to one of those components Once it cools down and shrinks back down it works correct How close to the ocean do you park the car or are you in the bay area where it gets really foggy ?
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)