[alan6272]

I have found on some vehicles that a head gasket can cause what you describe. A head gasket can leak some combustion gases into the coolant passages . A vehicle may run at normal temp in idle but once driven and under load it can cause the temps to run high. A poor water pump can cause this as well but you said the heater works fine so I would rule out water pump as this is what sends coolant to the heater core. There is a way to test if combustion gases are entering the radiator, with a die in a canister over the radiator opening. This is a specialized tool and die system that maybe your local radiator shop should have. Ask if they doo and if they can test this for you. Keep us posted.