this is possibly the volume control deteriorating. the mf volume controls on the older models aren't that great. just a possibility.
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Possibly a failing capacitor that eventually "forms" after voltage is applied for awhile. Might be able to locate it probing with an analog ohmmeter (preamp powered off). While you're under the hood, watch those high voltage power supply electrolytics; they can hold a charge for a long time if no bleedoff is provided.