I recently got an iPhone 6 (second-hand, so presumably out of warranty) and I'm having an issue. After unlock the screen, after a few seconds, the screen begins to get obscured by lines appearing both horizontally and vertically, until it's full of white/black/blue lines. I have tried resetting the phone by holding down the power and home buttons, and I have tried restoring from iTunes on my MBP. Neither has resolved the problem Underneath this issue, the phone still works, but it's impossible to know what's going on on-screen. This is temporarily rectified by locking the phone and unlocking it again, but then it does the same thing a few seconds later, even if I leave it on the lock screen. I know someone who will fix the screen for me, but could someone explain what the heck is causing this? Is it a gpu/logic board issue, or a screen issue?