Welcome to just answers my name is Kevin
either the radiator is clogged up or the motor has a head gasket or warped heads
The entire system was flushed so not sure how the radiator could be clogged and it has been tested twice for a bad head gasket and twice confirmed no problem. When the problem first surfaced, it would overheat ater 20 to 25 miles. After replacing the fan motor, thermostat, water pump, belts and radiator cap, it worked fine until a long trip. It did not overheat until after 125 miles of continual driving. Wouldn't a clogged radiator or bad head gasket mainfest itself much earlier?