Try this -- but be VERY careful not to touch anything on the monitor or any other live power source.
Get a piece of sheet metal from Home Depot or wherever, and cut it down just small enough so you can slide it in between the monitor and the power supply. It doesn't need to be perfect, this is just a test. See if it affects the monitor; move it around if you have to.
If this does lessen/eliminate the distortion, then you've figured out the problem. You can try the coffee can idea above, or you can build a shield out of sheet metal using a vice and some rivets (or spot welds if you are so equipped). You may need to use a couple layers of metal depending on how strong the magnet is.