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my phone is restarting every 10 to 15 minutes when it has the sim inserted, the sim that im using is a microsim to sim adapter from iusacell (mexico). when i updated to 15.3.A.1.14 i thought that this problem was fixed, until I restore the phone to factory data reset this problem shows again. this issue apears when i'm playing some music, or when im trying to unlocking the display.
I think it is SIM related.Get a mini SIM and see how it goes.
I agree with Alexdon that this sounds like a SIM problem. But you could always try to boot your phone into safe mode to see if the problem may be related to any 3rd party app that you have installed in your phone:
http://talk.sonymobile.com/t5/FAQ/How-to-boot-your-phone-into-safe-mode/m-p/348008#U348008