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It could be a notification app, a third party messaging app, taskiller, antivirus, etc, at this point it will be trial and error looking for the culprit.
Most likely a third party app causing this, boot on safe mode and report back
http://talk.sonymobile.com/t5/FAQ/How-to-boot-your-phone-into-safe-mode/m-p/348008#U348008
It could be a notification app, a third party messaging app, taskiller, antivirus, etc, at this point it will be trial and error looking for the culprit.