The problem with steel is that it will eventually become magnetic on its own in the face of those fields. This may be less of a problem if the field goes away with the pinball machine off, but I'm not even sure of that. I don't know if a rest period will reduce any magnetism that the steel has built up, or if it just delays the effects.

How is it that you know that the top-right distortion is not also the pinball's fault? However, assuming it's not, and you can't find any other source, it may be your electrical wiring. Does your wiring run through a wall near the CRT? This can be a problem in the US, and I can only think that twice the voltage is bound to make it far worse.