My phone started this a week ago after a system update. SO FRUSTRATING!! This happens once, twice, sometimes three times in a day and will restart itself 3 - 6 times before it stops. Help!!
Try the instructions to place your device into Safe Mode.
If the issue quits, then I would start deleting apps atarting with the newest downloaded to see if an app or an app conflict is causing it.
If that does not fix the issue, then try removing the SD Card and power up the device and see if that corrects the issue.
If that does not work you may have to do the dreaded Hard Reset
Be sure to save any pix and documents and backup your contacts prior to doing the hard reset.