If it were me and I was running out of ideas I would first resort to trying the system with just 2GB of RAM installed since you are using XP just to see if it alleviates the problem.
That operating system simply isn't made to handle that much memory and it does create headaches for some people.

Failing that I would then remove one of the video cards simply to see if maybe the issue is crossfire related.

Some quirks aren't easy to find and that's just a part of computing that we have to accept.

I've had some drive me nuts for ages before finding a cause or cure.

I'm not a gamer but can't help but wonder if perhaps the problem is simply related to settings in the game being maybe off.

Stirfried--
I do apologize for losing track of this thread...I have had a longggg work-week and my dad's in the hospital.
Could you look in the Windows event viewer-- in Admin tools ...
and see if it shows any errors in the applications log?
One or 2 would be "normal" ... a string of red X's is not--

Also --Start/Run type "msinfo32.exe" (without quotes) hit enter.
under hardware resources highlight "conflicts/sharing"-- if any conflicts note them-- and post here?
Check "forced hardware" in the same way, please.