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Are you using the correct SIM card size? does it happen at a particular time? does it turn off or does it reboot?
since this doesn't happen after you use an app, you might have to repair the phone with PCC, and report back
Bridge (for Mac)
Alternatives on How to backup Xperias
http://talk.sonymobile.com/thread/36355
Hi jele,
Welcome to the community,
Do you have an SD card inserted in the unit, if so can you try removing it and see if the issue continues?