Retested with zypper install kernel-default-5.13.2-lp153.2.1.g89416ca.x86_64.rpm Switched off Secure Boot. The upstream kernel loads. Log in. Leave the PC alone for approximately 10 minutes and it reboots itself. I now believe that it reboots after a period of approx 10 minutes of inactivity (so not random). There was no stack dump in /var/log/messages. I will retry a few more times. Is there a way to force the dump?