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I would suggest that you 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
If the behavior still persists in safe mode I suggest that you repair the phone software using PC Companion:
http://www.sonymobile.com/update
In PC Companion, press start on Support Zone > Start on Update the phone/tablet software > Press the blue link that says "Repair phone/tablet".
Before repairing the software in your device you may want to backup your information first. Check out this topic for more information on how to:
http://talk.sonymobile.com/t5/FAQ/Backup-amp-Transfer/m-p/407253#U407253
