I've just been sent a replacement 1020 as mine developed a screen fault. I'm starting to wish I hadn't bothered...
The new one has been hanging about twice a day, maybe more. Sometimes when the screen is off, other times when it shows the lock/clock screen - it just stops updating the screen and cannot be unlocked. I have to soft-reset (power+volume down).
It is a a brand new phone with no SIM yet installed, upgraded to 8.1 Denim as soon as it detected Wifi. I allowed it to restore itself from my Live ID 'backup' and added a screen protector - that's it.
Does this seem a slam-dunk faulty unit, or could it be something I can address? The phone is not under warranty (I sent it away for a replacement screen but they sent me a new phone without charging more).
update: I also did a factory reset and didn't restore a previous backup, this has not helped. And I now recall that it actually hung right out of the box when I plugged it in to charge before doing anything...