I am looking into it. Normally the board slows down in the early morning once a day while the server backs up the messageboard database, compresses it, and saves it.
I suspect it is our web server (which is nearly 5 years old now) which is struggling to keep up with the traffic and all the options on the new board software. I built a new database server last year which is plenty fast but now it is time to replace the web server. Or it could be something minor like having to restart apache.
I think I may have found out what caused it... We were deluged by a huge number of spiders and bots crawling the site and the utility I have in place to monitor them may have been overwhelmed. I disabled it and the server seems happier.