| Did your Pots go through the cover? if so you have replaced their ground plate with a nonconductive wooden one, Get hold of some copper tape or aluminium tape and cover the inside of the cover with it, make a jumper lead to connect the taped cover to a common ground, or the wire you mentioned before. If the pots do go through this cover, their bases should make contact with the metal tape to provide their common ground.
All of your cavities should be shielded covers included, and all of these shields should be connected to common grounds this should fix the problem. | 
Electronics Buzz 
