

Aaahh... a problem very near and dear to my heart. In our case, other
monitors cause this problem - the deflection coil of other monitors to be
specific. Have also seen a monitor backed up to a fuse panel exhibit this
problem. This sounds like your problem since flourescent lites, motors, etc
several 10s of feet away seem to me to be too far to cause it, but the
juice running to them must pass nearby your monitor. Fusebox on other side
of wall maybe?
We started spec'ing Panasonic CT-1331Y video monitors (3 switchable input
lines(vid & aud) S-VHS on one) <$400. This stopped the wavy interference
effect on the computer monitor next to it. 
Now on to your problem. You need what is known as mu shielding (very
common, in fact almost mandatory on electrostatic deflection type
O'scopes).
I talked to a EE prof. He said get a coffee can, cut both ends off, mount
around deflection coil of interfering monitor. 
BE CAREFUL TO AVOID ALL HIGH VOLTAGE CIRCUITRY. ESPECIALLY THE THICKER HIGH
VOLTAGE ANODE LEAD USUALLY COLORED RED. IF YOU KILL YOURSELF, DON'T
BLAME/SUE ME!!! USE PLASTIC OR OTHER NON-CONDUCTING STAND-OFFS AND SUCH TO
MOUNT CAN.
Now, I would assume that what is good for keeping mag fields in is also
good at keeping them out, so hopefully this'll work by mounting shield on
monitor being interfered with as well. If not, start shielding those other
sources.