I have a Hanns G 28 INCH monitor. When I play a game or RUN a program that has a dark background or dark atmosphere the monitor dims down right away on it's own. I have a dual monior set up with a 22 on the other SIDE but it doesn't have that problem on the 22 but I game and watch movies on the 28. Running a 260 GTX GPU and a ecs 6100sm-m 1.0 mobo running xp 64 and 4gb ram.You may get more answers to your problem if you can express the idea in other words. The monitor may be running normally. Do you meant the light level goes down, or does the contrast go down?
The monitor that does not show this behavior may be the on that is not working right. Good quality monitors are supposed to lower the light leve on a dark scene. Parts of the dark scene should be absolutely black. That is a feature, not a defect.didn't quite understand but if this sounds better... If I am playing Fallout 3 and I talk to someone in the dark the monitor readjusts itself and makes it even darker when the picture is dark. If I turn on a light the screen readjusts itself to the normal lightness. If it is a feature it is very sensitive and if a feature I need to know how to turn it off or adjust it...please Does it happen only with this one game?

no. it happens on any game or program that is very dark. Just dims even more. for instance, I have this .txt program that the background is black and the text is white. If I am full screened with that the monitor dims. as soon as I shut down that program it instantly BRIGHTENS up.