Another possibility at this point: Head gasket leaking combustion chamber pressure into water jacket. This could be what is blowing thru the cap and the overheating could come from the water pump filling with combustion gas and not pumping.
If it only fails on the road, you need to find a shop which has a detector which can sense exhaust gas in your overflow tank. You could also, cold or hot engine, crank it over with the ignition coil disconnected and look for air bubbling into the overflow tank. Good LUCK!!!!!!!