My 97 Honda civic temperature gauge goes up and down?

I changed the thermostat and its still doing that

Bleed the air out of the cooling system. You still has air in it…

Radiator leak or a possible warning of an impending blown head gasket.

The temperature gauge will go up and down on any car. Is the cooling fan actually working as it should? If not the fan may be defective or the thermal relay may be bad. Your car is 18 years old. Time for a complete cooling system service. To bleed the system, fill the radiator to the top, start the engine with the radiator cap off. Let the engine run and watch for air bubbles escaping from the radiator. Keep adding coolant to keep it full. Once you see no air escaping put the cap back on. Now look for bleeder valves located on the upper part of the cooling system. If you find any slowly open them to release any air. If there's none then you are done.