It could be the power supply as previously suggested; it could also be the backlight circuit as well. If the LED backlight driver circuit is weak and failing, this circuit is most likely your culprit. Bad capacitors could be at fault in either circuit.
If you’re inclined to do so, take the back of the TV off and look at the boards; check for capacitors that have domed or swollen tops, and might be leaking. If you post pictures of what you see, that MIGHT help others help you as well.