Here's the rundown.
There is a PHP coding error in the template of the forum, related to one of the modifications from the standard SMF software.
Those errors go into a table called log_smf_errors
They fill up at a relatively predictable rate.
Generally, activity increases on Friday evenings, which causes the logs to spike, and the error logs fill up the server to 100%, at which point it crashes.
If I remember to delete the logs, I do it, and the server doesn't crash. If I forget, usually someone reminds me on FB when it does go down, and I take care of it.
I could write a cron job easily to delete the logs automatically, but I would really rather have a real FIX, instead of a WORKAROUND.
That's the gist.