Temp gauge is acting wierd

[BookS13]

Ijust installed a new thermostat today because my temp was going up but only like once a week,after installing it i took it for a test drive and it started to overheat so i turned the heat on and it went down but there was no coolant leaking when i got back.After leaving work tonight i let it warm up for a min a took off as soon as it started to warm up all the way it wouldn't stop until i turn the heat on and it went down and i turned itff and it was fine all the way home. I thought it was the water pump but still showing no leakage i dont know what else it would be

[MikeMurphy]

maybe an air bubble?

[NISTECH]

yup air pocket. try raising the front of the car when you pull the coolant bleader screw from the intake manifold. it helps the air from the heater core escape during refill.