At the risk of triggering the players vs. collectors flames, it's probably dirt/wear in the pots. As suggested, you can try shooting them with contact cleaner + many rotations and see if that fixes the problem.
If you have a '64 Jazz Bass that has the original pots and untouched wiring from the factory, then changing the pots will lop hundreds of dollars off the collectible value of the instrument. The factory pots have date codes that reflect their date of manufacture and which would be something that was checked by a collector. If you replace the pot with a new one, then you should make absolutely sure to keep the old pot to help retain some of that value. You can also replace it with an old pot from that era, although that may cost a shocking amount (and you may well be buying somebody else's scratchy pot).
If the current pots are not the originals, then changing them at this point won't have any effect on the value.
If you love playing the instrument, then change the pot and don't worry about the effect on the resale value (but keep that old pot!).
David Fung
