Pope may be on to something to check....I changed one of my trucks to serpentine...went to Chevy and bought a reverse water pump and put it on....the thing would idle for hours here in Tucson with no problem. It would run at low speeds with no problem. But, at about 50mph, it got hotter than a dickens. I tried everything...one day I was leaning over the grille looking at what to do next and realized that the air was blowing out the grille, not being pulled in. I had reused the old fan....wrong direction. Sitting still, the air thru the radiator was going the wrong way but it still worked....going down the road, the fan was trying to push the air forward and the ram effect was pushing against the fan. Simple stuff, big pain.