I am having the same issue with my Certified Pre-Owned Note 3 as well. I have performed a system update, cleared the system cache, booted into safe mode, booted with and without my MicroSD inserted, booted with and without the SIM inserted and performed several factory resets, the device continues to freeze and crash seemingly at random.
I've been having the same issue since I got the phone two months ago. I got a replacement phone a few weeks ago after Tech Support had me do a hard reset THREE times! I've had the replacement phone for a week and it started shutting off and turning back on again!!! Yesterday it shut down and will not turn back on at all! I went to the Verizon store instead of going round and round with Tech Support and they ordered me yet another phone! I love the phone but I'm getting sick of it not working!