Sutter at 40MPH?

You need to add more details related to the problem, like where do you feel it, can you do anything to make it change, or does it occur all the time. With no other information to go on, this reminds me of a common problem in most vehicles caused by using the wrong transmission fluid. The lock-up torque converter will grab too aggressively, break free, then grab again repeatedly, causing a shudder. This only occurs around 35 - 40 mph and you can make it stop for a couple of seconds by tapping the brake pedal with your left foot.