The problem has been identified.



The photo doesn't show just how much of a lip there is on the spindle... enough that the cotter won't fit flat and snug against it without rotating and allowing the crank arm to slip backward and forward as you apply pressure when pedaling.

Well, crap. Guess I need to locate a replacement spindle or source a BB replacement. Can't imagine how I missed this (guess that goes to show what happens when you just presume everything is fine... I'll be inspecting things a bit more closely from this point forward.)

In the meantime, y'all enjoy your Stellas, Harps, etc. I'm sticking with Guinness.