This is a very interesting line of conversation. as some of you may know i breed my first litter of boxers just last month and i had the exact same thing that jed describe to me happen with one of my boys (he was under 1 week at the time), it was whilst feeding him, and he just went stiff, i thought he had gotten some milk in his lungs and had stopped breathing or something, but with a couple of shakes he came back, it was very scary. The weird thing was he was fine after that but we later lost two pups from a lung infection caused by milk inhalation, and the pup that had gone stiff was fine. He is now the smallest of the pups, but he is doing well, and eats fine.
I would be very interested in if anyone knows what this is, as when we told our vet he said it must have what we thought as it happened when i was feeding him.