Catching real cause of android reboot
You can try to use adb logcat command from the terminal/command prompt to examine continuously your device log. On your device has to be enabled USB debugging in Settings -&gt; Developer options section.

Or after the random reboot try 

su  
cat /proc/last_kmsg &gt; /mnt/sdcard/last_kmsg


from a terminal window on your device. You can download terminal app from Play.

For the su command your device has to be rooted.