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Repair your phone with SUS
Update Service (SUS)
http://www.sonymobile.com/gb/tools/update-service/
http://www-support-downloads.sonymobile.com/Software%20Downloads/Update_Service_Setup-2.11.12.5.exe
Bridge (for Mac)
Alternatives on How to backup Xperias
http://talk.sonymobile.com/thread/36355
Don't forget the Kudos
It Happens Due To Some Errors In Phones Frimware.
You Just Need To Re- Install It Using Pc-Companion Or Sony Update Service
Or Give a try by doing factory reset. (Dont forget to back up your data).
May be some third party application causing trouble,Before doing update do a safe mode boot so you can identify if its because of any third party app, still problem persist you can do a Repair using SUS as mentioned above...
Sony Update Service