My 1994 5.2 Dodge B250 van would run cooler below about 50 mph but would start overheating at hwy speeds, tried cap thermostat. water pump and poured water into radiator with bottom hose off and water came out so i though the radiator was ok so finally i got a radiator off a van like mine and it was a used one but the temp now runs as normal as a new vehicle even in hot weather or hills and traffic. The cores can still flow water but be restricted with build up and slow flow down which keeps it in the engine longer than normal, so at higher speed it causes temp problems.
