> Why are they at different potentials?  Are you using a single point
> grounding scheme?

Joe,

   How could you guarantee that the grounds at each end
of a long run would be at the same potential?  Each
connection in a circuit has an associated voltage drop.
Each wire has a voltage drop that increases with each foot
of length.  They are in the millivolt range, but they do
add up.  

   If the drops along the ground path are different than
the drop across the shield, you have a ground loop
circuit.

   The purpose of the shielding braid is to "trap"
signals so that they are not picked up by the wire inside,
or be transmitted to adjacent wires.  Grounding one end of
each shield will accomplish that purpose.

Regards,
Dale R.
COZY MkIV #1254