iPhone 5s Touchscreen problems

Hi guys, need a little advice with my iPhone 5s.

Over the past few weeks while i use my phone it occasionally turns off the touchscreen. By this i mean that the screen goes a little dark but is still on, and the home button still works, letting you open the quick access and go back to the home screen, but other than that you can to anything, swiping doesn't work, tapping doesn't work, anything that requires the use of the touchscreen is completely unresponsive.

Locking and unlocking the phone restores functionallity but only for a little while before the phone does it again. This is also very inconsistant, sometimes going dark moments after unlocking the phone, sometimes it wont do it for hours, but it is very frustrating when it happens.

Is this happening to anyone else? Is there a solution to this issue?
I've had the phone since early this year, bought it second hand off a friend, hadn't had any issues with it until a few weeks ago when this problem arrose.

Thanks guys