Have you had an software update recently Or have you installed a new app? Try shutting the phone off then hold Volume up-Power-Home. This will take you into recovery mode then just use the volume down button to highlight clear cache partition once it is highlighted then press power then volume down to say yes then power again. Then after it clears the phone will go back to recovery mode then just press power when the phone says reboot. This is not going to change any settings on the phone just clear out all the old files the phone does not need anymore. Has your battery been dying quicker then it did before?
I would be alarmed if my phone was restarting by itself as well. KATHY65793 does raise a good point. Have you had any recent software updates? Has there been any liquid or physical damage to the phone? When did this start? Are you having any issue with charging the phone?
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