I have seen radiators get hot at highway speeds and run just fine around town. It didn't matter what the temp was. The core gets pluged up and can not transfer heat fast enough at highway speeds. A stuck thermostat or removed one will normally cause a cold engine. It takes a lot of work from the engine to heat up with out a stat.

If the water pump set with a blade or two in water for 30 years, it could be bad, or the clearance in the impeller is worn. But at higher rpm's it should pump more water than at idle and your problem would be reversed.

I'am betting the radiator is plugged and the problem showed up at the same time as the thermostat went bad or the weather changed.

Joe

1937 Chevrolet Inline-powered Streetrod