I've encountered odd problems like this before with big screen tubes which
are as Jerry says particularly sensitive to magnetic fields including the
earths own field, radiators which may have been spot welded and are now
effectively bar magnets and even, some yrs ago, my mothers Hyundai car which
when parked 8 feet away through the wall caused purity problems on their
TV- different depending which way round the car was pointing.!

It may be possible to place a compensating magnet somewhere around the set
(experiment to see if this works for your set) You will only need a small
magnet like the one from the speaker of a small radio. I've had sucess with
this in the past.

It is also worth a reminder that TV's only degauss themselves when powered
up from cold, not from standby soa few sessions of 5-10 mins unplugged then
powered up may help the set 'acclimatise' to its surroundings
AW