Ok, so I've had the car for a little over 2 years and now that the temp outside is coming down a bit I noticed that my car isn't getting warm. When I turn it on in the mornings the temperature gauge goes up but when I start driving and turn the heat on the temperature gauge goes down. It's done it a few times this year but only when it's colder outside. If I turn the heat off the temp gauge starts going up but it never gets near the middle. Has anybody had this issue before? The check engine light isn't on and it's a 99 GS300 thanks in advance.

 

The thermostat is going bad.