Have you by any chance got a GeForce 2 MX card?
Just I was at my friends house today playing it and I had the same problem as you're having.....On Curse][, do the walls change color and go brown etc in some places? and do black squares sometimes appear on the floor?
I have a Geforce 2 GTS, not the MX.
I do have this changing color problem, walls change from this greenish color back to their normal color as I advance through eg. this corridor:
I did a 2nd install of UT which gave the same problem, I also increased my cache from 4 to 64 mb, didn't work. But after some checking and clicking through the advanced options the problem seemed to have dissapeared. Might be that I clicked empty cache, I need to check again tonight...