1989 Plymouth Voyager Temerature sensor

I replaced my radiator, coolant, and hoses. Since then my temperature sensor needle fluctuates violently. I do get heat. Whenever the needle becomes steady for a few (10-15) seconds, it appeaqrs to indicate correct engine temperature. But the needle never becomes steady for a longer duration, like a few minutes.

What did I mess up while changing the coolant system? Do I need to change the thermostat or the temperature sensor or anything else? Could it be a loose wiring? I want to resolve this problem before it becomes a major headache. Right now my my 6
mile commute in town, it poses no problem

Thank you. I appreciate your help.

Manny