usually after about 30 ~ 45 minutes into playing an Onslaught Map. I can play the same Map in single player mode and never get a crash (or freeze up I should say). Yes I'm playing overclocked but it's only those Maps doing this. All other games are fine in Multiplayer.
Yea try to reinstall it. I recently had issues with a couple of my games and a reinstall was all it took to get them back up and running.
If UT2004 is crashing and your PC is OC'ed, chances are that you need to increase voltages or drop your OC down a bit.