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I had the same problem on my 25KC45, and like Bryan said, it turned out to be the resistor pack on the luma module. Resoldered the pins which were obviously cracked, and touched up a couple questionable ones, and it's been behaving well since. I've still got an intermittent slight contrast dropout, caused by a dirty pot on the IF subchassis, but haven't bothered to pull the set apart to get at it just yet.
The biggest problem on those sets is the module pins - if you remove the modules, clean the contacts and use a straight pin to add a little tension to the contacts on the modules (there's a couple fingers which point inwards to contact the chassis pins). I used a pencil eraser and contact cleaner to clean the chassis pins. You'll probably wind up breaking a couple of the little blue tabs; you want to keep as many of them as possible, since they are what holds the modules in, other than the tension of the contacts.
Good luck!
-Jim
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