It's one of two things, either it's near a magnetic source or something in the Mario Widebody, or more probable is that it needs degaussed. Sometimes when you move them (especially when they're turned on) it'll do that to the monitor. You need a degaussing wand or ring to clean it up. You just run it around it with the power plugged in and it resets kind of how the stuff is working inside the tube. I think you can carefully do it with a magnet or soldering gun too, but do some searching and see what you turn up.
Sometimes it goes away over time, too, since the monitor has a built in degaussing coil which zaps it a bit when you first turn it on.