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Does this problem occur if SIM is not in the device? Once every two or three days is not that much though. Maybe you could check how is in safe mode:
http://talk.sonymobile.com/t5/FAQ/How-to-boot-your-phone-into-safe-mode/m-p/348008
hope you get sorted
As Alexdon says, try running it in Safe Mode for a while....Safe Mode will show up any 3rd Party App that you have installed that may be interferring with the smooth running of your phone, if the problem is still apparent whilst running it in Safe Mode then you know it is not a 3rd Party App.....then you can look at other ways of fixing the issue :smileysmileythumbsup:
Software repair is the last step,first try safe mode.