Hi, Doug. Is it an LCD, or a CRT (tube) monitor? These comments are for the LCD (LED) type of monitor. If it's getting dimmer all over and not in any kind of pattern, it's probably the backlight. The game has no effect on the backlight, but it may have an effect on the pixel matrix. Doesn't sound like that is your problem. I assume you've gone into the monitor's setup and changed things like the brightness and contrast settings? If so, your monitor may simply be wearing out. Of course, excessive use of any monitor can prematurely age it. Do you shut it off when it's not being used? I had an HP 20" LCD burn in pretty badly once with some Ham Radio software I kept running. It corrected itself after I shut it off when I wasn't using it actively. Got better after about 5 power cycles.
Don't know if this helps, but something to think about.
Best,
Marc
BTW - if it's a CRT, there's very little hope for improving a dimming image.