Do you mean that the device restarts on its own, or that you force a restart due to a freeze or something along those lines?
A good start would be to see if the same occurs in safe mode, and narrow it down from there. If it restarts automatically in safe mode, could you try without a SIM and SD card to see if the same occurs? If it instead freezes while in safe mode, try to see if a software repair using Xperia Companion helps.
