Just went through this exact problem with my friends 79' Chevy stepside. It turned out to be the relay for him. It has to be either the relay or the temperature sensor. I was able to figure out if I took the relay out, hit it on the side of the truck, and plugged it back in, it would work fine. A new relay solved the problem.
If I were you, I would just buy a whole new relay and temp sensor. Depending on fan amperage, jegs has some for $40. This would save you the trouble of finding the problem and finding the correct relay/sensor to fit the current system.