I had this strange crashes in all modes of the game, yet with a very stable system outside UT2004. I could finally narrow down the problem to the memory module that I had recently installed.
How to check only CPU+MB+RAM?
Simple! Just download the prime95 client from http://www.mersenne.org/freesoft.htm and run it. When you run it for the first time, select the "Just stress testing" and then in the menu, in options select the torture test and select the "Blend" test and run it for at least one hour.
If you have any problem with your RAM timinig, CPU or MB over heating or anything else, this test will give you an Error! This can happen even if all normal programs seem to run perfectly. In my case, I could run almost anything with no problem and thought that the crashes in UT2004 was either a driver probelm or a bug in the program. But my computer didn't pass the prime95 test and finally I found out one of my RAM modules was not that good. Replaced it with a good RAM (Kingston HyperX) and all problems magically were solved!