I'm stuck. Bought a 71' F350 motor home with a 390. 77,000 miles. It is overheating but not all the time. Drove up a 5% grade about 3 miles long and it overheated. Two days later, same outside temp, drove up a 5% grade about 6 miles long. No overheating. I have replaced the thermostat. The radiator is huge and very clean. Running 50/50 coolant. The timing is on and the advance pin in the distributor is still on. (had a 390 lose this before and overheated constantly) The water pump feels tight when I loosen the belts (no slop) so I don't know where to go from here. It's a cool old motor home and I want to keep it on the road. Any suggestions?