I disabled lock screen, so did not have the symptom you have.Mine wasn't a direct flat black screen, but it kept flashing between lock/normal screen for a couple of minutes before restarting by itself!?
It had been rock solid for a long time under Android v4.2. Think v4.4.2 need some time to stabilise. But actually I did not see any significant difference between the 2 versions also. Really hope to see firmware update real soon to address the issues.