It sounds like EIC (exercise induced collapse) to me. Dogs collapse when they get super excited. They don't lose consciousness and will flounder around with their back legs not working. After a few minutes it passes and they are fine.http://www.thelabradorclub.com/library/eicstudy.html
I would look into seizures. run just about every test you can IMO. It has to be very scary to witness.
Could be something like even the excitement of eating triggering an episode.