iPhone 5S rebooting it self

Dear All,

My iPhone is keeping rebooting several times a day. For example, yesterday, I had 14 times rebooted. My iPhone is updated (8.1.3), and I don't know what else I could do. I tried to restore by iTunes (also with recovery mode), and even with only factory applications, it still presenting the same problem. I called to Apple, and they couldn't help me, telling to send my iPhone to Apple. But the point is: The problem starts only after I upgraded to 8.1.3. If exists one way to revert for 8.1.2, which was Ok.... but now Apple stopped to sign this version...

Anyone have any idea that what I could do? I really appreciate your help on this.

APPLE I'M REALLY DISAPPOINTED!!