Why do you believe the headphone jack is responsible? I think in 20 years we have seen between 1 and 0 headphone jacks that were bad from the manufacturer. I would be almost positive that this is a solder joint issue. We find that it is incredibly common to see one of the two black wires that connect to the headphone jack just get completely missed when soldering, and this will cause a lot of distortion and other weird problems.