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Try to start your phone in safe mode to see if the problem may be related to any 3rd party application installed in your phone:
http://talk.sonymobile.com/t5/FAQ/How-to-boot-your-phone-into-safe-mode/m-p/348008#U348008
If you experience the same behavior in safe mode I suggest that you repair your phone software using PC Companion:
http://www.sonymobile.com/update
In PC Companion, press start on Support Zone > Start on Update the phones software > "Repair phone".
If the problem still persists it sounds like you should have your phone examined at a service center. Please contact your local Xperia support team for more information: