Need help. '70 Mustang fastback. 351W with pertronix II ignition and Flame Thrower coil. 1.5 ohms no ballast resistor. Drives great for about a half hour and then just shuts down. Like the spark immediately fails. Pop the hood, bang on the coil a few times with my knuckles, fires back up after about ten minutes. Is the coil overheating or what? Thanks for the help.