Default [Q] Screen switches on and off at regular intervals

Hi,

This morning my S4 started to continually activate the screen for about 5 seconds then deactivate it for 1 second and repeat the cycle continuously. This happens even when I press the power button to shut down the screen or leaving the phone idle and when charging/not charging.

I have switched off all the smart screens, motions and gestures and I have done a hard\factory reset with no change!

Would you have any ideas what's gone wrong or is it likely to be a hardware issue.

Thanks