Most of my reboots happened when I was in Safari or Settings (oddly enough), but I have had a number of app crashes. Waiting to see if 8.1.3 improves this any (although I'm still getting lag frequently enough to be annoying, and I have had at least one app crash).
my first phone did that randomly, once in a while then it got to the point that was doing it 2-3 times a day, I took it back and got a new one, never had a single reboot.