2002 Mercury Mountaineer overheating. Changed waterpump, thermostat, fan clutch, flushed radiator( running clearly through)and change hoses and clamps. Radiator cap new and tank not leaking. No leaks visible, no coolant lost. About every other week overheats to HOT. Not idling just normal driving conditions. No smoke from exhaust. No clicking in engine. Everything sounds and runs good except for the extreme overheating. Please help, don't know anything else.