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does the problem show up when you are in safe mode?
did you perform a factory data reset when you software repaired your phone?
try to see your network settings, sometimes when the phone encounters super low signals it reboots, don't know why though
/t5/FAQ/How-to-boot-your-phone-into-safe-mode/td-p/348008
safe mode will open your device to only the apps that are preinstalled with the phone thus allowing you to konw that the problem you are experiencing is not caused by that apps you installed.
so if the problem doesn't show up in the safe mode, most likey you have installed an app tha causes the problem,
keep us updated 
then it is likely a hardware problem. you can have your phone check at your local service center http://support.sonymobile.com/global-en/contactUs/