?
Try putting a small magnet around the area
taped to the back of the crt untill the spot stays still.
not to strong of magnet, to strong will change the purity
to much..confirm your degausser has no ac running thru
the coil after being on..after a minute at plugin..
leave the game on and disconnect it at the monitor only.
the shadow mask maybe moving alittle.
because of age.
stick your meter probes down the connector and monitor the ac
getting to the coil.
Confirm the ac is turning off and staying off.
when its warm it opens the circuit when its cold it close the circuit.
and allows ac thru the coil.
Bad connection to the posistor or shadow mask moving?