Well this is embarrassing ..

I thought it may actually be my Leica IIIc causing that trouble (I shot that roll about two months ago.) So I ran a roll through both cameras.

.. and the problem did not resurface

I also tried Rick Olson's CRT shutter speed check (http://rick_oleson.tripod.com/index-135.html) on both cameras and neither showed trouble (as did two other unrelated cameras.)

Sorry everyone ..