I faced the same problem on my Nexus 5 recently (after more than 2 years of usage). I was able to fix this problem by opening the back panel and pressing the flex cable (which might have gone loose) on connection areas. In some cases the flex cable might have got damaged resulting in this issue. If that's the case with your phone (i.e. if even after the above procedure, the flickering persists), you can try replacing the Flex Cable. Check "https://www.youtube.com/watch?v=4pks4idq... for flex cable replacement.