Only overheating on the freeway.

Crusty radiator, my 66 Belvedere got this way even with a 3 core.
Took the top off myself and rodded out the crud, and soldered the top back on and happy.
See if you can find the old oxalic acid cleaner that is two step.
The second step neutralizes the acid.
If not, buy a gallon of CLR, add to system with distilled water only.
Run it 2 days, be prepared to change water pump afterward.
Dissolving the crud is easier than rodding out a radiator, even if you have to change the water pump.