From your description the tube has to be magnetized somehow. This is typical in corners especially. Being close to walls with electrical lines, speakers, other appliances, all kinds of things can create a field which can magnetize the tube, sometimes over a long period of time. If I were in your shoes I would either acquire or make a degaussing coil to cure this. You might also try using the TV in another location, sometimes this alone can fix things. And deguassing, by simply turning off and on. The degauss coil in your TV is very weak, so while it can fix your problem, it might take many, many hits, hundreds maybe, and areas that are farther from the coil may be less affected--like the corners. This is why a handheld deguasser is so wonderful, you can solve the problem in a minute or two instead of cycling your power 100 times.