I picked up a 90s duo sonic that sounds great, but has problem in the wiring, and it randomly loses signal going out based on movement of the jack cord (or tone pot?) the signal gets cut by ~25% and sounds super thin but still comes thru. I twist the tone nob and move the cord around and it comes back to 100% (acts like a short, but it’s signal loss). I had a shop take a look, and they said everything seemed fine, but replaced the jack as it was very flimsy. and it’s still happening. they said they checked the pots and wiring and all seemed ok, but it isn’t at all. when played clean it so faint I couldn’t even demonstrate it for them. I’m at a loss, any ideas what would cause this?