I've had something similar since swtiching to windows 7, I'm running UO from and SSD on a nice enough machine so I know it isn't the machine slowing in some way.
I also have a very sensitive VPN running on the same connection, so I know my conn hasn't dropped, or that would be the first to go, but yeah once in a while I lock up completely. (once every couple of hours probably, can also be twice in 30 mins or not at all all day)
The difference for me is that I don't come back, maybe I don't wait long enough but I'm pretty sure I should have got the client crash message. I've noticed that I don't get this any more. UO just locks up and says nothing till I close and re-load.
Are you running Windows 7?