What is it with Americans coming up with new words? Use either shudder or stutter for crying out loud. Studder????????????????????
Sounds like the car just hesitates intermittently would be a better description.
Did you guys even go to school - or did you forget most of what you learnt when you left? It's not hard to speak like an adult.
For what it's worth, I'll try to help. A modern car takes miles to adapt to a higher octane fuel. From my experience, the consumption of around 3 tank fulls of a higher octane fuel allows the car to fully adapt to it and therefore make the most out of it. If you are exepriencing the hesitation symptoms described here, run you car on three full tanks of the highest octane fuel available and see if the problem persists. Don't try one tank of low octane in between the other tanks of high octane.