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This is normally caused by faulty Sim cards or SD cards - You could try safe mode to rule out an app issue - if it stops then it's an app but if it continues then you may need to try or get a new sim card
You can either manually downgrade or return your device to the place of purchase or contact Xperia care and have your device inspected http://www.sonymobile.com/global-en/support/contact-us/