1960 Valiant Restore: advice welcomed.

Some like it hot, some not. So, I pick up Mabelliene in time for the local car show, and she starts overheating. First the temperature gauge stays normal, then it slowly climbs to overheated range, then slowly works its way back to normal and then goes slowly back to overheating.

This seems like a thermostat issue, but they tell me they replaced it when they refurbished the radiator. If not the thermostat, then what??? Is it possible the water jacket, that being the ducts for block coolant, are clogged solid, too???

Needless to say, I didn't attempt to take her to the car show.