When I am driving under acceleration in 3rd, 4th, or 5th (around 2500-3000) I notice a very suttle hesitation in power. The engine and drivetrain sounds perfectly smooth. It feels kind of like if you were driving and you lightly and quickly pumped the brakes 4-5 times, then it stops and I might drive another 50 miles before i notice it again. It is more noticeable on hills and under accelleration. I should also note that the plugs were recently changed at the dealer. Any ideas??