- Do you have sys-rq enabled and if so does the system respond to an emergency sync then emergency dismount then emergency reboot? [0] sysctl -a 2>/dev/null | grep rq
- If you switch to run level 2 does this still occur? [1]
- Is there anything interesting in syslog messages in /var/log? Most notably Xorg.log and messages
- Do you have lm-sensors installed to watch temperature? sensors | grep -Ei ^temp Also check the temp of your drives smartctl -x /dev/sda | grep -Ei ^temp assuming your drive is sda.
That would be my starting point. It could go a million directions from there.
[0] - https://en.wikipedia.org/wiki/Magic_SysRq_key
[1] - https://www.tecmint.com/change-runlevels-targets-in-systemd/
open a console and type
watch -n 1 "dmesg | tail -n 40"
to watch the last 40 lines that are reported by dmesg (some distros will require you by default to be root to do this, so if you get an error, try that)
Also, you may find information in system logs:
A traditional (non-systemd) distro will store text files under /var/log - often /var/log/messages is what you're looking for - you can watch the tail as above:
tail -f -n 40 /var/log/messages
If your system is running systemd, you would do:
journalctl -f -n 40
(again, you may need to be root to do this)
I recently had lockups that seem to have been thermal in nature - there weren't a lot of related messages - just once I saw something about hitting thermal limit, but that was a bit before the lock-up, so I dismissed it as just the behavior of modern cpus which run full-tilt until they reach thermal throttling. The machine was sent in for repair and had liquid metal thermal paste re-applied, and temps are down by about 10 degrees - really not that much, and temps are still sitting in the 90s, but so far, over the last 1.5 days, no lockup, but I'd need to run for a while longer to verify.
Perhaps check that your BIOS is the latest version first.
And by the way run also a scan of the RAM with memtest.