Hello I have this problem for a long time. I got iPhone 6s installed with iOS 9.3.4 and I saw that the lines are flickering whenever I change apps. Do you encounter that problem? and do you have any fix for that?
The few display issues that arise mostly due to app compatibility problems. The flickering screen sounds more like a hardware fault. It could be something as simple as a loose connecting cable, but it is more likely to be a faulty LCD screen.