I suggest Odin flashing the stock NC2 firmware and then factory resetting from stock recovery. (Backup first of course!) If it is still an issue, it will most likely be a hardware problem. From what you have said so far, I believe that you will find this to be the case. Could've been due to damage from a drop, or even overheating at some point. Or it could just be that its simply due to hardware degradation over time.