Could be a number of things. If it's not rooted, I would first suspect the sd card. Try pulling it if you have one and see if it stops. If it doesn't reboot with the sdcard out, try formatting the sdcard with the phone, or better yet, try a different card.
Another thing to try would be a full factory wipe and reflash with either kies emergency recovery or odin afterwards.
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2012 ASUS Nexus 7
- KitKat 4.4.4 Stock ROM
T-Mobile Galaxy Note 3
- KitKat 4.4.2 Stock ROM(TW)