Why does my iphone 5s randomly restart by itself?

My iphone 5s has been randomly restaring for about 2 weeks now on its own. I tried resetting my settings to see if it would fix the problem, but it didn't. The phone is new. What can be causing this? I'm running iOS 8.3 btw

My iPhone 6 has been doing the same thing. I know there was this bug that could be sent via text, but at this point I don't even get the text and it will randomly restart. What's weird is that I usually unlock my iPhone with the finger print scanner thing and if my phone dies or I turn it off and back on you have to type in the password because it won't let you do the finger print thing the first time after it has been turned off, but when it restarts itself I can use the finger print scanner no problem. So its like my phone doesn't even know it was reset or something. I'm pretty sure Apple is attempting to fix the bug because I've had it for at least a week if not more and I know many more have complained. So we'll just have to get through it for now. I think the bug is only in either the most recent version of OS X, or its only within the iPhone 5/5S/5C and the iPhone 6/6+ because I've never heard of someone with an iPhone 4/4S having this issue.

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