Thermostat or not

Question... Does it run perfect early, then spark rattle under load after warmed up ?
If so, pony up for a new radiater. I had the very same problem with a 67 years ago. If caught in a traffic jamb, you'll overheat it. Been there done that. I even used the heater fan to try and cool the water. The final straw for me was leaving a concert. Boiled over before We could get to the open road. The most expensive part of the cooling system so naturally the last thing replaced. LOL