My first guess would be a bit of corrosion on the volume control. If so, an easy fix would be a touch of "tuner spray." Or otherwise, you could try replacing the volume control, which could get a little expensive.
My second guess would be a spotty capacitor somewhere, given the age of the unit. Capacitors become suspect in 20 year old electronics. The bad cap could be hard to identify, so it could be a trip to the local service shop. By the way, after they locate the bad cap, I would urge you to consider having that cap replaced in both channels, since the other channel could be close to retirement also.
My second guess would be a spotty capacitor somewhere, given the age of the unit. Capacitors become suspect in 20 year old electronics. The bad cap could be hard to identify, so it could be a trip to the local service shop. By the way, after they locate the bad cap, I would urge you to consider having that cap replaced in both channels, since the other channel could be close to retirement also.