I have a 1993 Chevrolet s-10 4.3 v-6 model truck. It overheated one night. The gauge shot all the way up to red and then all of the sudden dropped back down to normal quick. The heater core was already going bad. So we replaced the heater core. While replacing it we also saw the water pump was bad. So we replaced it too. Also replaced thermostat and all hoses. Well my son drove it for one week after that with no problems. Then all the sudden the gauge shoots up again. He also replaced the cap with a newer easier to open cap. We then replaced the temprature sensor. We dont know if you have any ideas as to what it may be. Our last three options were blown head gasket, clean and flush radiator, and the clutch on fan. Thanks for any ideas. We also replaced the thermostat and replaced again becasue we thought we may have received a bad one. Also the radiator is like 4 years old ( we bought new ) and has had nothing but antifreeze in it. Thanks again for your time.