Xperia Z1 Reboot without even touching it

I just got my xperia Z1 from Sony service they changed my motherboard.. i opened it and it was working perfect! i downloaded some apps used it for like 20 minutes and after i locked it and didnt use it for like 2-3 minutes it started rebooting it self and they said its a brand new Sony Xperia Z1 Motherboard... please help me and thanks for your time :)