I have a Squire Vintage Modified Jaguar Bass (the one with the black body and matching headstock) with a stacked concentric volume and tone pot for each pickup. The stack for the neck pickup keeps fading out on me. A slight tap on it often clears it up, but I don't want to gig with this bass because of this problem. I have sprayed it with contact cleaner a couple of times - but this didn't fix it. What causes this? How can I fix it?