Hi @Rumu, let me see if I can help out here.
Your post has been translated to English as this is an International forum, please post any replies in English and it will be easier for me and other users to assist you.
Has this behaviour just started to appear or has it been going on for a long time? Also, have you been able to relate the behaviour to a specific use or does the phone turn of randomly?
The first thing that i would suggest is to reboot your phone in safe mode to see if the phone turns off there as well. Safe mode is a troubleshooting mode where all apps downloaded from Play Store is temporarily deactivated, that way you can see if this behaviour is caused by a downloaded app. For more information on safe mode and how to use it, click here.
If the phone still turns off while in safe mode, I would suggest that you do is to try to perform a software repair using a computer and Xperia Companion. Please note though that a software repair will erase all user data, so remember to do a backup of your user data first.
Try this and let me know how it goes!
