Strangest issue here. My car overheats only when driven on the highway for extended periods of time, quicker with the a/c unit running. Only during the summer. Ive changed out the head gasket twice, water pump, thermostat, radiator cap (OEM), flushed the entire system. bypassed the heater core thinking it was clogged. Run the car without a thermostat, still the same problem. Cooling fan comes on pulling air through, there is no blockage of air flow, system is purged of all air.. what gives?? only thing I havent changed is the radiator but would a faulty radiator cause this?