Spray some electrical contact cleaner in to the gap where the terminals are on the potentiometers on the inside of the unit and and turn the pots end to end a few times.
I recommend De-Ox-Id as your contact cleaner. I use it, and find it to be a good balance of high effectiveness and low price.
https://www.google.com/search?q=De-O...utf-8&oe=utf-8