Why is my screen flickering when phone gets hot?

I have this problem with my lumia 925. When I play games/watch youtube and after some time my phone gets really hot, screen starts to flicker (it changes brightness and contrast really fast). Usually it happens when I try to play gta san andreas for more than 10 minutes. It also happens during youtube videos when charger is connected.

My phone was at Nokia Care two times and they haven't repaired it.