Hitachi tv is flickering

Hello I have an hitachi 42HYT42U and the top left of the screen flickers, I've had it for about 3 years, when I have my backlight on low it flickers, when it's on medium it flickers, but when it's on high, it doesn't flicker but it turns off randomly after a couple minutes, does anyone know the problem (model wasn’t there so I put a random one)