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Just curious , have you replaced R77 and R78 with new resistors ?
It sounds as if one of them is drifting in value as the set operates , and just because they test good at room temperature is no assurance they are not drifting with the application of power .
If you have replaced them , and recapped the rest of the circuit , hopefully one of the site's resident TV experts will step in here for ya with a suggestion or few ....
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