Thanks to all who replied.  Don Lewis steered me in the right direction.
Turns out I was using UTP cable with solid wires, and crimping with RJ-45
connectors meant for stranded UTP.  This results in unreliable crimps, and
can cause shorts or open circuits.  Beware, Radio Shack sells solid UTP, but
only seems to carry connectors for stranded.  Lesson learned.

Cheers,
Mark