My engine temperature goes up when I 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

Check to see if radiator fan is running when the temp starts to go high.