I dunno. Seems like it could be a problem with your graphics card. Is your computer getting enough ventilation, or have you cleaned it out lately. Your graphics card might be overheating. Happened to one my my friends old PC's. If that don't fix it, your card might be dying or, as the people above are saying, it could be an old monitor that is just dying.
tried checking the gamma of the screen? its like located in control panel somewhere in visual settings/ something of the like.
i was playing wolfenstein's enemy territory and it messed up my monitor badly, so i went and asked a friend to help out and he says to mess around with the gamma.
try it, maybe its not because of the monitor. i have an old school one, and its still working fine.
Check the screen settings are normal (gamma, contrast, brightness, etc). If that doesn't work, make sure the unit's well-ventilated (even try getting a vacuum cleaner to the vents and cleaning it). After that... it's probably faulty. If it's under warranty, replace it. If not... get a new one.
But this *is* a Dell product. They're cheap, efficient... and prone to fault.
Most screens have an auto button which will ajust it to normal settings and resolution to fit your PC settings. If that doesnt fix it, I would purchase a new LCD screen and hook it up to see what happens, I can say almost surley, your old dell screen is dying.