2000 Quest temperature gauge goes down while driving

[yasimshd2008]

Hi, I have a 2000 Nissan Quest SE. From a few months I have been noticing that the temperature gauge goes down while driving on the highway. In normal idle mode it warms up normally and even the heater works fine but suppose im cruising on the highway around 50-60 mph the temperature goes back down to C (cool) and if the heater is on it also doesn't let out hot air. At first we thought maybe it has something to do with the radiator. But it has no leaks. Its filled with regular radiator fluid. Can this be problem with the thermostat ?

[far raf]

What's the ambient temperature when it does that?

[amc49]

Stat barely sticking open to not close when needed to.