Samsung tv 60” overheating issue

Hi, I have a Samsung 60” tv and it seems to be overheating. It would have lines running down the screen after 5 minutes and it was quite hot. I replaced thermal paste and now it will last 30 minutes until the lines form. I still felt as the tv was hot so I opened the back cover off and turned it on. Worked fine for the duration I watched (about 2 hours). I put the back cover and within 5 minutes the lines formed again. I quickly took off the back cover and the cpu on the motherboard and tcon board were both very hot to the touch. Also the metal plate holding everything onto the tv was extremely hot. How can I fix this?