My first 2 instincts are...Call Nissan USA, as well as finding another dealer to check it over. I've never, never seen a vehicle do something that crazy. Acting up like that would initially cause me to think something is loose through the drivetrain. But you can fully see your RPM's instantly rise as soon as the shudder stops. You need to fight with that dealership more if you insist on taking it too them. Considering it's the same issue, I think as long as you have a lemon law, I would say this fully qualifies based on the amount of times you had it in for the same problem. Keep us informed of what goes on, but stand your ground with those guys.