OK, I had the Lumia 928 for a few days and unexpectedly the lock screen froze and was unresponsive. I had to do multiple soft resets and finally the screen unfroze. I don't think heat was the issue for me. I may have come across something that is worth noting.
I am very detailed on going through all the phone settings and take note of turning things on and off, etc. When I got the phone, I noticed that the "Find My Phone" setting simply had a toggle switch for "ON" or "OFF". When I toggled the switch to ON, it displayed on. When I pressed the back arrow key to return to the settings menu, the text under "Find My Phone" said off, when the setting was actually on (confirmed on by going back into the setting and the toggle switch showed on). I thought this was just a display glitch in the software and didn't think much of it.
I recently updated to the Amber Update and GDR2 update. I thought maybe the Find My Phone setting display would be corrected but it wasn't.
I then decided to do a hard reset wiping all data to see if that would correct the screen freezing. After I did the hard reset, I went to settings and this time, the Find My Phone setting did not have a toggle ON and OFF switch, but had two check boxes sending apps to phone using push notifications and save my phone's location periodically... This was definitely a different Find My Phone setting after a hard reset than when I got the phone out of the box. I also had a stranger lock screen instance - when I swiped up, the froze for a few seconds showing half the screen underneath and flickered pixelated colors for a few moments then unlocked.
Anyway, I went to Verizon explained the screen locking issue and they gave me a replacement phone (within the 14 day policy). When I set up this phone, I went to settings and noticed that the Find My Phone setting was the same as after the hard reset on the previous phone: two check boxes sending apps to phone using push notifications and save my phone's location periodically... NOT the simple toggle ON and OFF switch that did not display correctly!
So, I am going on a limb here and blaming the screen freeze on some bad software installation on my first Lumia 928 phone. The replacement phone has worked flawlessly.
For those of you having the screen locking issue, check your settings and see what your Find My Phone setting options are. If it shows the simple ON / OFF toggle, then maybe this is a potential culprit to this issue.
hope this helps.