Just an update. Nikon service returned the camera without fixing the problem a second time around, even after tech support recognized the problem. Once again, it appears Nikon service went through their standard series of diagnostic tests without bothering to actually test whether the camera body still exhibited the problem.
I am convinced that it is an early symptom of a failing shutter. Now I have uploaded new images at Nikon's request and will once again send the camera in. In my personal experience, Nikon service is pretty lame - nevertheless, I will keep sending the camera back (on their dime) until they fix or replace it - thankfully, it was originally sent in on a warranty repair.