When coils get hot, they expand like anything else. If there is insulation bad in the coil, the windings will expand to where they ground to the can or the metal part of the coil. It usually starts with an occasional miss when it is hot to where it finally gets real bad. Condensers are usually dead or alive and when dead stay dead. Coils come back to life when they cool off. It was common enough that my father kept spare coils under the seat of our trucks that we delivered fuel out in the country with. My first guess would have been vapor locking because it's the most common. My next guess is the coil because they also fail when they get hot. Be interesting to see what develops tomorrow. However if it still does it with a new coil, you mention the shutoff valve under the tank. There is a plug on the bottom of the petcock. Remove it and see if any trash comes out. This is the spot where trash likes to settle. Be very careful when you remove the plug. Hold the petcock with a vice grip or something. There is a brass 1/8 pipe nipple about 2 inches long that goes into the tank and it is very easily broken. With the plug out, root around in there a little. Oh, by the way. something else that can happen is the wrong gas cap. These vehicles were designed for a vented gas cap. I remember putting an unvented gas cap on a school teacher's pickup. He drove most of the way home and had to walk the rest of the way. We followed him and when he was out of sight of his pickup, we put the correct cap on and I suppose he never did figure out why his pickup quit.
