I’ve checked the last journal, but unfortunately there were not much details in it. I guess journal entries were not synced to disk when the error happened. You can try to change sync interval to every 5 seconds, instead of default 5 minutes, to see if we are going to have more info.
Edit /etc/systemd/journald.conf, uncomment SyncIntervalSec line, set 5s, instead of 5m and reboot.
Some other things to consider: did you overclock your CPU? Have you tried using the LTS kernel?
You are saying that you can reliably reproduce the issue, so with little luck we should be able to track it down.