I've had two, both were when I was in the car with the phone connected to my bluetooth system (in car system) and connected to a Motorola Fast Charger.
I've never had it reboot while actually using it or on a call etc..
No reboots for me, but I had to pull the battery after it froze ...guessing it was a 3rd party app that was also draining my battery (both improved after uninstalling it)
i've had a couple. 1 was while just standing at the gas pump. 1 was after unplugging it from the car charger. that was my fault tho. when you take it off the charger it turns the screen on. i hit the lock button at just the right time when the screen was coming on and it freaked out and rebooted
1 time it just shut down...was on the charger and was getting texts. started playing some COD Zombies. about an hour later i go to use the phone and when i hit the lock button it goes through the boot sequence. was a bit worried by that but it hasn't had any problems since. this was Monday or Tuesday
I don't have any type of app killer on my phone but today i drove home without it being on the charger and it didn't reboot - maybe it is something to do with power?