I agree on the field neutralizing circuit. If it's the shadow mask, then temperature should affect it. Have you tried putting a big fan behind the set to cool it down, and see if the mask pops back into shape? I've seen a 40" Mitsubishi CRT do that, although it had been clobbered (loosened the mask from it's mounts, or something). as the set warmed up, you'd hear "Boi-oi-oi-oingggg", and the purity went to heck. You could also give it the fist in front, and it would look like a stone thrown in still water with color ripples.
That Westinghouse got slammed around in a rough ride in my TV room in '94, and nearly fell off where it was sitting. I found it wedged between the wall and a console set, and it had been shaken up pretty badly. A couple of the knobs that fell off, got under a file cabinet and were crushed. If the mask is dislodged, blame the Northridge Quake
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Did (present owner) give you the correct volume knob? I don't remember seeing it on the set last time I was at his place...
Charles