I've been running 20180105 firmware, and there was long period with very few reboots (once in a month), then suddenly there came one week with 6 reboots. And now last week, no reboots. And no changes on usage, full time 4G mobile data on.
Try to update to the latest OTA firmware, after this update my phone has not been restarted by itself.
The latest OTA firmware for me is: 2018-03-07, if f when you try to update the device the answer is "the device is already up to date" try to switch to 4G and click the "check for updates" button again, for me worked!
And remember to perform a factory reset after each update to avoid weird bugs!
Requiring factory reset on software update is really stupid thing. Think if your windows would wipe you disk totally empty on every security update. Nobody would tolerate such. Re-install all apps, change settings again. And worst of all, authentication apps (google authenicator, RSA SecurID, etc) need re-activation, and in most cases that means calling to some service desk (Amazon, Google, etc) to get things rolling again.
Updating software should be normal operation, just click of button. Factory reset means one days work for me. And for any security minded people, updates are not option, they are mandatory. If updates aren't installable easy way, they won't get installed, and device will be unsecure.
I just updated my phone with OTA, let's see how things will go.