Sony Oled XR48A90K Dimming problem

Have a Sony OLED XR48A90K and after 30 seconds to a minute it starts dimming and the only way to un-dim it is to go to the menu and back in for it to brighten back up but this only last for another minute before it starts again. Really hard to capture on camera as its much more obvious in person but if you pause it and click between 3 seconds in then 8 seconds, you can see it instantly brighten up.