I have had random reboots before the Android 6 update and after the update they are still there.
Am I right, when I guess it is a hardware problem?!
There are very many possible causes and workarounds for random reboots.
If none of the tips from the first post of that topic help you then - no matter the android version - the right thing to do is contact support.
As this is no “other” reboot problem I’m closing this thread, please continue in the other topic.