I have a 94 Ranger 6 cy 3.0 liter that only gives heat when it wants to. I have tried more than once to burp the system but no luck. The idiot gage does the windshiled wiper thing, up and down, up and down. When this happens on the high side I get heat, on the low it gets cold. Any help out there?
How old is the thermostat? The engine temperature should not fluctuate. Once the engine reaches operating temperature it should stay there. Try a new thermostat and radiator cap.
Is the cooling system completely full and free of air?