Why is my temp guage going from normal to 3/4 and almost overheating? on 1994 Toyota Celica

When I'm driving my temp gauge will start to climb to about 3/4 or more but not fully overheat. Then I will put the heat on and it will drop back down to halfway which it normaly is always at. I checked my anitfreeze level and that's good. Radiator is newly relpaced. Is it my thermostat?

Asked by for the 1994 Toyota Celica
ck coolant level to start with if good ck cooling fan operation