Default [Q] Vivid turns off hard

I have an ATT Vivid, SOff, rooted, TWRP installed, running DU. Runs great, but every now and then it turns off. The battery has plenty of charge, not running any apps (often in its holster). I go to wake it up and find it clear off and not responding to power button. Pulling battery works.
Any clues as to what is going on here?
Many thanks in advance
LG Optimus V - ICS, now retired as an alarm clock
ATT HTC Vivid - S-Off, rooted, unlocked, KitKat 4.4