Replied to a similar topic in the root forum, but reposting here in case it's of help to anyone:
My brother, my girlfriend, and I all bought this phone and all use it similarly and live in the same area
Of our 3 phones, only my girlfriend's exhibits this reboot issue. It's pretty random when it happens, but at least twice happened when we were hitting device storage writes hard (downloading music via google music), and once the phone entered a boot loop when picking up a call.
We exchanged the rebooting phone and now all 3 phones are fine.
This is all circumstantial, but it does seem like this is a hardware issue that requires an exchange.