Monitor flashes colors in standby mode

I have a Dell Dimension E520 that I purchased in 2006.  It has a 19 in. LCD monitor.  When not actively using

computer, it goes to screen saver after a certain time, then after a few minutes it goes black which is

normal.  Recently it has gone to screen saver and then instead of going black, it starts to flashing different

colors.  At this point you can not push the off button on the monitor and turn it off.  Does anyone have any

suggestions?