My engine temperature goes up when I come to a stop

Hello everyone, Ive enjoyed reading the this forum and it looks like there are a lot of knowledgable people here. I have a 97 toyota rav4 with a problem. When I come to a stop, my engine temperature goes up to the H, but when I start going down the road the temerature goes back to the middle. Does anyone know what this might be? I just ordered an oxygen sensor because my car needed it but I am not sure if this is the problem. Thanks for anyone who takes time to respond to this

thermostat, fan relay, or fan itself.

Yup. My guess is that the fan isn't kicking on. Once the car is moving the fan is all but unnecessary. If it were the thermostat, chances are it would overheat all the time.

Also, thermostats 99% of the time will fail open.

My bet is the fan.

Yeah, my vote goes to the Fan motor, relay, or sending unit. A multimeter should make quick work of this problem. Good Luck.