iPhone 5S - Unresponsive (Blackout)

Over the last month or so I have been having issues with the responsiveness of my iPhone 5S.

It will sporadically blackout (no image on screen) and become unresponsive to button presses, being plugged into charge and my MacBook will not register the device when it's plugged in. Once the phone comes back on these things all work fine.

The first time this happened, my phone didn't respond to anything for about 5 hours before randomly coming back on (still with a nearly full battery life). Since then, it happens randomly and could last for 10 secs to anything up to about 5 or 6 hours.

Anyone know what is going on or having similar issues?

Thanks,

James