Coloured patterning is more usually due to a slightly magnetised screen. However, in this case sounds like the set may have suffered from condensation affecting the high voltage connection to the tube. Often there will be signs of arcing around the tube rim or tube anode cap. You may also hear the faint fizzling sound of high voltage arcing. Had a Philips TV last month with the same symptoms you have described. If it has been in a damp environment, sometimes they can be dryed out but would need PAT testing for safety but after a year it's probably scrap.
Cobweb
September 2004
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