monitor contrast changes with window colors?

has anyone else seen this? my monitor (dell 1704FPT1) sometimes suddenly dims and goes into a mode where the contrast of the entire screen depends upon the colors in the foregrounded windows (Win XP). if there is a lot of black/dark showing, the entire monitor's contrast goes very dim. if i click over to a window with more white, the contrast returns to normal.

i'm not sure this is a backlight issue, since if i bring up the monitor's (hardware) menu, it stays the same brightness/contrast regardless of what is going on in the background.

this 'mode' is intermittent, sometimes suddenly coming out of it and everything returns to normal, where the screen bright/contrast is not affected by which XP window is up.

help?