I get on UAL.com almost all the time and yes, sometimes the site is down when I get on. But when I get back on and it is up again, there is usually a change in pricing. So I am assuming that they may be updating their servers to change pricing, update E-Fares, tally up and send out account balance info for Mileage Plus members and general cache cleaning. But yes it is frustrating for someone who wants fares ASAP and cannot wait more than 30 minutes to get them. That happened to me once and I just flew American. I later checked and UAL wound up being cheaper by only $11 or something like that. Oh well!
Its better to be on the ground wishing you were in the air than be in the air wishing you were on the ground. Fly safe!