My Phone are changing from bright to darker

[Ravendoux]

I have a Xperia X. 

And when i texting in messenger or is in my webbrowser my screen gets darker and after a second or two it gets brighter. 

I have checked in my settings and everything is as it should, i have no battery savings on, it is on full for brightness and I dont have the option on for automaticly change The brightness level. 

I have no idea what to do. 

Please if anyone have an idea please help me. 

[_alexdon_]

@Szandor Have you checked if the same occurs in safe mode?

[Ravendoux]

No i have not. 

Thanks for the tip, i reply if it worked.