[nuttle]

This seems to be a common symptom, and there are probably many different underlying problems. In the case of 3504, the round cable (from prior years' KoP) was the issue. It seems to have signal integrity problems for any digital signals that change very quickly or that may not work properly in the face of noise.

It probably isn't a bad idea to keep the distance this cable has to span reasonably short and follow the suggestion in the instructions on how to rework the ribbon cable and shorten it if you're doing the rework anyway.