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Hard to tell, you could test the phone without the SIM and SD cards for many hours and see if that's the cause, but it'll be wise to send it to your Local support to get it inspected.
This is what I would do to narrow down the issue, repair the phone with XC > do NOT install any app > remove both SIM and SD cards > use the phone like that for many hours and see if you experience any reboots, if you do then send it in, if not then insert the SIM card and test it, then the SD card.