Uncontrolled reboot

Greetings!

Since updating my system from openSUSE-11 to openSUSE-13.2 my system reboots uncontrolled after 3-5 days.
There is no hint in the system logs (journalctl), just the plain text ‘-- Reboot --’

I checked the BIOS for possible temp control, but there is no such feature.

I also checked the disks with smartctrl but also no sign of a problem.

The previous system ran w/o problems.

If you have a hint how else to hunt down the ‘rebooter’, I would appreciate it very much.

Wolf