after this car is warm, at idol it overheats and cools off as i go down the road again. the repair shop changed the waterpump, $500 bill. still overheats. they tested for cracked head or blown headgasket. that looked good. brand new radiator in february along with thermostat. they replaced the thermostat again. it still does it. they said they tested the fan and fan clutch and they look good too. now what? any ideas?