The worst EMF effects come usually from sources immediately adjacent to the CRTs: fans, in-wall cables, microwave ovens, other CRTs, space heaters and the like, rather than overhead power lines and even more remote sources. The behaviours at different locations that you describe seem to rule local effects out. You've probably gone as far as you can with substitutions, and, if you have now isolated a prime suspect (board? yoke? gun?), you are left with a replacement of the offender.

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