Everyone who has replied has had good advice. Fuel or fire will fix most problems. Check and see if this car has a ballast resistor on the cowl. Basic ceramic block with resistor inside or Ignition module. Either will exhibit the symptoms you described. Out of the blue...no reason...will run and stop. Days or minutes. Swap and go in most cases. I had a 77 D150 for many years. Carried one each in the glove box. Truck had a mind of its own, but ran forever...the ignition issues notwithstanding. I am betting ignition as others have mentioned.