The latest and greatest is that MySQL was running out of file descriptors. The fun thing about that is that just modifying limits.conf doesn’t fix it, one needs to modify the systemd start-up script for MySQL as well as the /etc/mysql/my.cnf.
The re-install blew away my modified systemd scripts and so what I had in my.cnf was ignored as it was greater than the system default of 1024.
This is fixed now. I also did some tuning to hopefully speed it up a bit. Shaved about 100ms off o the page load time.
Now I’m waiting to see what blows up next.