Yikes! Random reboots are no good! Let's get this fixed. Does the device reboot while in a particular application? It sounds like the software maybe corrupted. I would highly recommend a complete restore of the device http://bit.ly/Sq8I9x .The restore will attempt to repair the software and return the device back to factory defaults. How long have you had this device? We have a 14day WFG return policy http://bit.ly/IeiJI . Keep me posted.
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My iPhone 5 has been doing the same thing. Just started last weekend. Since then it has shut down unexpectedly at least 6-7 times in the past five days. Sometimes while I was talking other times will just on standby. It was never due to lack of a charge. Each time I am having a harder time getting it to reboot using the softboot (holding the two buttons down simultaneously). Can't get through to service/support now either...