How to separate the screen from the frame?

Hello I've bought a Z4 tablet from a friend but after using it for a while one part of the screen starts to flicker. Its like its duplicating other parts of the screen into this rectangle wich covers like 1/4 in the lower left. If i turn it off and leave it for a while it dissappears but when I've then used it for a while again it comes back. The longer i use it the more intense the flickering/flashing becomes. My first thought was that it was software related but I've tried repairing it through xperia companion without succes. I'm now starting to believe its a hardware issue. Im thinking it might be the flex cable that gets some kind of a reaction when the tablet gets warmer. I'm considering to try to open it and replace the flex cable but after searching on the internet it seems to be a big risk that the screen will break and the cost of a new screen assembly is way to high if it does. So the question is could this be a problem of the flex cable and if so what is your advice for separiting it with minimal risk of the glass breaking? I've found a heat pad separator for around 50 $ wich im thinking would help alot and i can have use for it later too but the question is what temperature i should set it to and for how long.