Since yesterday my Nexus 5X has gone into a reboot loop, eventually I get prompted for my password

Since yesterday my Nexus 5X has gone into a reboot loop, eventually I get prompted for my password and it boots normally, and then a little later (could just be a few minutes) it goes into the cycle again. It might reboot 10-20 times before getting to the password prompt again.

I got frustrated enough to factory reset it last night, and then when I woke up this morning it's doing it again, so I guess my next test will be to revert to Nougat and see whether it's better. I hope it's not a problem with my phone :-(

I must say I'll be relieved to return to Nougat for a while though:
- the dozens of noisy notifications have been driving me bonkers, and turning the sound off for all of these unimportant things is laborious.
- since Android O upgrade bluetooth doesn't reliably connect to my car (2016 Toyota Auris).
- saying "OK Google" starts the Google Assistant and then the phone stops listening to me.

Edit:
Google Support have identified this as a hardware fault and will be sending me a replacement phone.