You would have had to turn it on manually by editing ultimatebb.cgi. The included defaults turn off the board at a very high load, so if it is indeed turning off every few minutes, that server is simply getting pounded...
Your Recent Visitors time limit is set to 60 minutes - that's just nuts. Cut it down to 15 minutes, that should improve performance majorly and give a more accurate view of how many people are REALLY on your board at once. Once we have that figure, we can continue troubleshooting...
