been troubleshooting weird overheating symptoms, at highway speeds its all good, sometimes runs a little hot so I turn on the heater for a few minutes to cool it down. but then as soon as I get off the highway and slow down it spikes to red. ive changed the thermostat, the waterpump Is not leaking and my radiator appears to be fine. so I think its the fan clutch not engaging the fan once I slow down from highway speeds. Do I need the OEM part or will a good part from Napa do? if so where should I get this part from. I live in camp Pendleton CA, any advice?