Intermittent black screen fault

Purchased an un used iPhone 6s that was an insurance replacement. Never been opened or used worked fine for about one week then started to turn the screen black for long periods before turning itself back on. Whilst this happens it still rings and can be answered if you know where to swipe to answer.

As it has.not warranty (this has been checked with Apple) we had the screen replaced but this hasn't made a difference.

Has anyone any ideas as to the fault and a fix