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It could be something neurological, like a type of seizure, but it sounds like he is aware during the episodes so it could also be pain or behavioral. These kind of things can be very hard to figure out. Try to write down when they happen, how long they last and any other things you notice. I would also get just a general health blood panel done. Most things that cause symptoms like this will not show up on bloodwork, but some might (such as a liver shunt). What breed is your puppy? Sometimes the breed will give us a clue.