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Settings-apps-all-smart connect,clear data.
Check if the same occurs in safe mode:
http://talk.sonymobile.com/t5/FAQ/How-to-boot-your-phone-into-safe-mode/m-p/348008
I would suggest you to try a software repair:
http://www.sonymobile.com/gb/tools/pc-companion/
This will erase all data from your device.
It could be a hardware problem.Stop by a sony service.
I am sorry but do keep us updated.