The usual cause would be dirty/tarnished switch contacts. I sound is restored when a switch is punched in and out, that would likely be the culprit. Get some Deoxit and apply a bit to each switch, or you can spray with electrical contact cleaner. Bad switch contacts are one of the biggest problems that you find with vintage gear.I recently acquired this decoder and have a question. What causes the sound coming from the rear channels to be intermittent?