I have just return my 2nd 920 for the same issue. Now the shop where I ordered it, is no longer exchanging them as the issue seems to frequent.
It is a very strange problem as it is not permanent, but seems to degrade with time. Soft/hard resets seems to fix it but only for period. It also some time fix itself alone (just waiting and suddenly the screen works again). I have tried to understand it and have a strong feeling that the touchscreen software is blocked by some other activity of the phone, in particular when the gsm network link is poor. For instance disabling FACEBOOK reduce dramatically the problem. I also noticed that the issue increase with the added apps. I suspect that some apps installed are also requesting data connection.
However the conclusion for me, also I have tried to remain with Nokia phone, is that I am now switching to the competition. I could not support them further. For the time being I'm back with my N8, while I found another smart phone for me.
JF