I would guess it's hardware, but you could try a hard reset and see if that helps. If you can't do it in the menu due to the issue you can do it with the buttons as explained here: Nokia Lumia 920 hard reset
Okey just on the side note. i am a reseller and i sold 2 phones and both had the same problem. Bought from singapore. I wanted to know anyway it could be fixed by flashing? 2 having the same issue is too obvious and happens one in a million. I mean both phones having same issue. Maybe they added the wrong firmware when manufacturing?