Lift either the shield from the "non-functioning" station or the EMAP for the stations. If then the stations function we know its a ground loop.
To correct this, trace the shield. It should be daisy chained from station to station like the rest of the data cable. Commonly this problem will exist if you have several of the shields under the common in the EMAP or all the shields tie back to one point. (i.e. star topolgy on the shield only)
Lifting the shield and leaving it that way is not an acceptable solution. Also it may seem there is not 2 shields per data cable. The tech may have to disect the wire to find that there are 2 discreet shields under the heat shrink that was used.