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Debugging mysterious freeze / crash


I've been experiencing a great deal of frustration recently with
intermittent freezes / crashes on my Debian Sid system (a Lenovo
W550s). The symptoms are that the screen totally freezes and the system
becomes completely unresponsive (even ssh attempts from another machine
fail), and the only thing that seems to have any effect is a hard
reboot (holding down the power button until the system restarts).

Upon reboot, I can't find anything at all interesting in 'journalctl -b
-1', or /var/log/syslog - the former just shows everything looking
normal until the moment of the crash, at which point the log just ends,
and the latter also just shows everything seeming to be fine until the
moment of the crash, and then shows the boot messages from the reboot.

Any ideas of what could be causing this, or how I could go about
debugging it? I've been using this machine for years without
experiencing anything like this, and I'm not sure for how long this has
been a problem. I did recently upgrade from stable to unstable, but I'm
not sure whether or not the problem's initial occurences coincide with
the upgrade.