My 14.04 Ubuntu laptop runs fine but restarts every 8 minutes.

It's as if some process is issuing a reboot command, rather than a hardware reboot. There is nothing of interest in /var/log/kern.log

Memtest showed no memory problems.

Later, I changed out all the hardware. i.e. I just moved the hard disk to a different laptop. The problem was seen even with this new system.

I've also tried kernel upgrades, but the problem persists.

/etc/apt/apt.conf.d/50unattended-upgrades does not have automatic reboots enabled.

Thanks in advance for your suggestions.

Hi...

One possibility is that a cron job was made by something. Please see this thread. If not and no one else has any suggestions, you might want to file a bug report with Ubuntu.

Regards...

Usually if /var doesn't have a clue then one of two things may be going on. On is the fault doesn't report to /var. Two is the fault is so dramatic that it doesn't have time to report. (assuming no cron)

I'd look at top maybe to start.

If the problem moves we can assume it is on that drive pretty much.

A full clean reload might be in order.