I have 1 1993 Chevrolet S-10 4

I have 1 1993 Chevrolet s-10 4.3 v-6 model truck. It overheated one night. The gauge shot al he way up to red and then all of the sudden dropped back down to normal quick. The heatercore was aread going bad. So we replaced the heater cre. 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. Also my husband can take the cap off with no boiling overwen truck is suppose to be all the way red, and also we have no heat when all the way red. We dont know ifyou have any ideas as to what itmay be. Our last three options were blown head gasket, clean and flush radiator, and the clutch on fan. Thanks for any ideas.