Got the car running finally. Installed a used 2002 motor, and a million other things. Everything needs to be replaced when a 1970 2002 sits indoors unused for 25 years.

Pulled the radiator, twice. Sent it to a reputable radiator shop, who cleaned and refurbished it. Replaced the thermostat, also twice. All new hoses. Coolant is passing through everywhere, all hoses get warm, so I don't think there's a blockage. Heater blows warm air.

Takes about 15 min of driving before gauge gets into the red, and then only about 10 min to cool down into the normal zone. Then it overheats again within 7 min.

What can the problem be? Would an air pocket in the system cause this?