I've got a 1979 Chevy Cheyenne, not sure on the mileage, but the odometers rolled over so its probably pretty high. Its got a 305 with a 2 barrel Rochester carburetor. I recently replaced the water outlet and thermostat, and ever since then its been overheating. It will get really hot, then cool down. It will do this once or twice, and then it seems to cool down and hold temperature, at least for a while. After replacing the water outlet and thermostat again, the problem continues. Before I replaced the parts, I had no problem with engine cooling at all. I don't seem to be leaking any coolant from the water pump, and its not noisy. What I have noticed is that when I hit the gas with the truck parked, the upper radiator hose will collapse. After its gotten hot and cooled down a couple times, though, the hose won't collapse and seems normal.