what happens is humidity/dust, Over the years can cause a potentiometer's internal mechanical connection to lose or interfere with its electrical connection, moving it back and forth mechanically restores a better electrical connection, and in the Classic and Classic II's fixes oddness, (sometimes)
Also what might be happening, Dust/humidity (COULD be) creating kind of a short, allowing abnormal RF into that specific circuit.

Showing abnormalities on the screen

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