First off the A/C causes the aux fan to run on low speed, not high speed. The resistor reduces current to make the fan run on high speed, so if your fan runs on low speed, you can eliminate that possibilty. The fan clutch locks up only when the hot air flows over it. If the radiator is not flowing properly , then the center of the radiator will not get hot enough, for the air to be warmed sufficiently to casue the fan clutch to engage. Your thermostat is most likely ok, or it would run hot at all times,ditto the water pump. Best guess is eitehr trash in front of the condensor or between the condensor and radiator, or failing that ...replace or clean the internals of the radiator, price is about 450-480 if you need one