I am the owner of a 1984 Monte SS which is all original except for and alternator, starter and pollution pump. Recently I noticed the temp gauge climb to 125 deg ‘C’ when the car is heating up from a cold start, this takes approx. two miles at which time the gauge returns to operating temp. within seconds and continues to run at that temp until next cold start. We have eliminated such things as thermostat, water pump, fan clutch, temp sensor, coolant level, cracked head. Looking for any ideas or suggestions.