When this happens, does your game crash and the crash reporter come up prompting you to send us a report, or do you end up just force-quitting the game? If the crash reporter does come up, could you put your screenname here in the message to send with the report, so that I can quickly find and look at it?
I looked at your debug.err, and I see nothing suspicious at all. I thought perhaps your graphics device was failing, but I don't see any warnings about that at all, or any warnings about anything for that matter. Is there anything that you do do in common between games where this happens, like alt-tab out of focus of the game, or play from a certain saved game?
If when your screen goes blank, you do get the crash reporter dialog and send us a report, let me know and I can find and look at it. If not, though, let's try getting some more info by going into your Prefs.ini folder and changing the following section to the values shown here:
If the problem happens again, post your debug.err here again, and hopefully there will be more info to go off of to find the problem.
Another thing you might try is disabling fullscreen (if that is enabled) or changing the game resolution.