And how do you know that the temperature is normal? Because touching with your fingers you will never realize if there is any component that is failing due to excessive temperature. Look, I had a problem with a daewoo tv that was just because of a smd diode that failed due to temperature after a while on. Eye, you can have a similar problem and the 5 hours is relative. Make a test, leave the TV without the back cover and place a common fan, whatever you have at home and that is releasing fresh air to the platelets. If it does not turn off then you already know that the problem is thermal and you will have to start sectoring to reach the component that fails. The other you can do is turn it on and while it is running, before it goes off, heat it with a heat gun. With care, not so close. The idea is to raise temperature and if the problem is thermal you will turn it off to the touch. Start there and comment to see what happened. Regards