One thing you might want to check is IRQs. I had problems with random freezes and crashes in multiplayer UT, and when I installed my new SCSI card, I noticed that my Prophet II, NIC, and SB Live were all on IRQ 11 along with about 5 or 6 other devices (USB bus, etc). After disabling my LPT and COM2 ports, the vid card and SB Live got moved to different IRQs and UT runs a whole lot better and I am not getting the freezes or crashes. If you have a COM port that you aren't using, try disabling it to free up an IRQ for something to slide over to and see if that helps.
-Tom