Is this using a digital connection (HDMI or DVI)? I've had this problem before on DVI, sometimes the digital signal can get out of sync and the screen will become jittery, flash, discolored, etc. All the times it's happened to me just turning off the monitor and turning it back on fixes the problem (the monitor gets to synchronize itself with the DVI signal correctly).
If it's using VGA it may be either the monitor or video card failing or it could just be synchronization. Restart the monitor in either case and see if that fixes it.