recently my 2002 mustang with a 3.8L has been missfiring/stumbling. it occurs when i am accelerating around 1700-2000 rpms at the end of a gear. the car will jerk a few times before it shifts. the car only has 35k miles on it. i was thinking of a spark plug change but it only has 35k on it. could i go to autozone for a diagnostic? what do you guys think?