The quick test for a monitor problem is to swap out into another system. The fact your monitor is 6yrs. old isn't good and I suspect that's the real cause, its just showing its age. There could be outside forces like the magnetics as another poster offered, don't rule out RF sources as well. if someone recently moved-in with a ham radio or new transformewrs, etc. these devices can cause havoc if not at least weird stuff. You can at least with proper common sense, remove monitor and blow out the dust build-up this too is a cause of flasky operation as a last resort. You're best bet as repair wopuld be too great is to replace if in fact the monitor is at faualt.