In June we had a problem with our CoLo host company. They basically went belly up and it took an attorney and 10 weeks to get our servers back.
During that time we were on a borrowed server; between setup, tweaking and the fact that it was not a barn burner, we saw our share of problems and shut downs. About two weeks ago, tyme reinstalled our own server into the new CoLo....naturally that took some tweaking also.
The basic problem we face now, as I understand it, is the database that vBulletin runs on (MySQL) and the manner in which vB performs searches. The search function is ugly and the database has not met its promise in terms of handling large volumes....that's why most sites our size archive old threads.