Sometimes when the phone is locked and if want unlock the screen, there is some flickering in the screen, i am not able to access anything in the screen, if nay call comes the phone is ringing but the lines appearing in the screen, this issue had been quoted by many 1320 users, you cal also check this in youtube https://www.youtube.com/watch?v=lZ_ik7lB7aQ.
This happens to a lot of Lumias. Happened to mine twice. Both times the phone was being used a lot and therefore hot. One other thing was that I had just turned on "double tap to wake screen" when it happened the second time, so I switched that of again and hasn't happened since (so far)