My wife's iPhone 6s has a screen that has started flickering- the colors look a bit odd as well. It looks like it is flickering between bright and not bright (as opposed to dim.) It also tends to lock up for a couple of minutes from time to time. She does not want a new phone, but it is very critical for her work that her phone is okay. Any ideas on this? My 6s+ (bought in the same purchase) is like new. Not sure what is causing this.