Chosen Solution
Phone was updated from iOS 13 to 14 and, since then, has begun to reboot every 3 minutes. Though there were no issues prior to the update, rear housing has been replaced and all parts are original except for the screen. Have attempted: Placing back the original screenRemoving the sim cardBooting it with all cables unplugged(including either the power or charge port cables)Formatting through PC The screen glitches just a bit prior to shutting down. Small greyer line in the bottom and flashes a bit. It looks exactly like a hard reset, actually. Performance is also noticeably slower. It doesn’t reboot when locked and idle. The phone is also peculiarly marked as a Demo model on 3uTools. No clue as to the origin. It will occasionally notify that the battery is not original, despite this being untrue.
There could be a number of reasons it is re-booting but if it does it around a set time it usually turns out to be a faulty charging port or battery. I would put the logic board into a known good housing to rule out parts but this is usually not an option for the everyday person so I would be looking at the battery and charge port assembly.
Had this very same problem with an iPhone X immediately following a housing swap. I contacted STS Telecom (Which I totally recommend for data recovery) and the technician informed me that the iPhone X will restart by itself every 3 minutes if the charging port is not connected or it is broken. I went on eBay and ordered a high-quality replacement cable ($10-$15) and viola it was working good as new! I know this may not be the exact problem, but I hope this helps! P.S From what I have heard, there is a temperature sensor on the cable and if the cable does not have that or it is damaged it will cause the phone to have a kernel panic and thus shutting the phone down. so make sure you have a good quality cable!
