Possibly overheating volvo

Hello all,
I purchased a 67 Volvo 122s wagon in February this year, since then I have been doing some maintenance items on the car. after doing all service measures, oil, coolant, new radiator, hoses, belt, plugs, rebuild distributor, valve adjustment and rebuilding the carbs… etc. it is all together.
However, now the cars temp sensor is showing it overheating while idling for 3 minutes. While driving, it will keep at around 3/4-5/8. We were tuning the carbs and the sensor shot way up but the car itself had no signs of overheating. When idling and revving the engine after it showed it was hot, the temp sensor shot back up to the middle position, almost like it was a direct reaction to tapping the throttle.
I am clueless on what the issue could be, there’s a new thermostat, and the temp sensor appears to be working well when we tested it in hot water.
Any advice is greatly appreciated on how to fix the temp sensor or if there are any ideas on what could cause the spike/quick drop in temperature.
Thanks!