Something is causing the ultimatebb.cgi to continue execution beyond the needs of the user session. I've been doing some moving of threads early this morning during a period where there is very little forum usage. Yet I noticed that my CPU usage again skyrocketed to over 20.0 on the "uptime" scale!
A little poking revealed that, although there were only a handful (around 8) of established sockets as reported by netstat, there was still a very large number (over 20) of ultimatebb.cgi processes as reported by ps.
If the ultimatebb.cgi process is not properly terminating, this may account for why CPU usage tends to rise rapidly.
