My best thought is you have some sort of poor connection that is affected by heat or arcing once the monitor runs a bit. There are a couple of ways to troubleshoot this. If you have access to another computer and can try to duplicate the problem on that one, it can possibly rule out a problem with the PC. If it occurs on another machine, it's the monitor. It might just be a bad connection in that the analog signals for each color are sent on their own wire within the cable. It could be the cable itself or something within the monitor such as amplifiers for that part of the circuit. You can try to old "concussion" test when this occurs (give it a brisk whack) or wiggle the connector cable at the video card (and monitor if the cable is not hard wired. A change in color when you do this might give a clue as to where it is. Cable can be replaced but one that are hard wired are a trick. The internals of a monitor are not for the everyday user. Most will say stay out of these so I won't instruct how to approach this area.