Hi, I got this problem after replacing a rear back glass with heat gun, I accidently removed the LED from the Power Flex Cable and the phone kept restarting exactly every 3 minutes. This is not strange because the phone checks for that specific cable every 3 minutes, also other genuine parts, if they are missing it will restart. You check this by going into General-Privacy-Analytics & Improvements and finally Panic. It shows that there’s something missing, that will be your culprit.
I replaced with 3rd party flex, still restarted every 3 minutes. So I soldered the old LED back on the flex, which I thought was broken because the LED literally cooked on the inside. For 4 days I’ve been failsearching this phone, literally braindead at this moment because of it. And the phone finally works normally, although it now fails to find Mobile Network… I restored the phone and can’t go pass the activation phase, I’m getting warned to contact Apple support, this is just ridiculous.
In Short: Power Flex Button culprit, 3rd party flex won’t always work, try find genuine or fix original. Sometimes Port Charger culprit, so literally anything that has a sensor and has to be identified by the IOS every “X” amount of minutes.