The radiator fan hasn't been off recently has it? If installed backward it will push air through the radiator from the engine side installed of pulling air from the grille side. It will cool well at idle but heat on the highway, the ram-air effect through the radiator is negated by the reverse air flow from the fan. I have seen this twice over the years with mechanical fans, first time my mistake, second time a customers vehicle. I have also seen this once with an electric fan, wired for a pusher when it should have been a puller.
John
