We've been scouring mailing lists looking for people with similiar problems.
I wrote a script quickly when it locked this most recent time to collect some info... learned a few interesting things.
Found a piece of code in LiveJournal that shouldn't be causing the lockups, but might be if the database is misbehaving, so we changed it around to make sure it isn't LJ.
We should have enough information soon to figure out what the real cause is, then get the developers of the database to fix it.
But, on the bright side--- we were doing 5 Mbps of traffic when the db locked. That's faster than it's ever been.