1997 Sunfire overheathing

Hello!

I have a question about my car. If sombody could help that would be great.

I have a 1997 Pontiac Sunfire 2.2 L 190,000 Km.

When I am idiling my tempreture gage will begin to go up. It does not go into the the yellow or red zone. When I turn my heat on and turn the fan up on high it will begin to go down slightly. Then once I begin driving again it will cool down as long as it is not start stop driving. I have just recently had my water pump replaced and my coolant is fine. Is this my thermostat?

Thanks! :)