Here's one, When I accelerate and reach a speed of about 40-50 mph I feel it shuttering it seems as if it's comming from the rear but then when I'm above that speed it runs very smooth then when comming down in speed I feel the same thing about the same speed 40-50 mph then it runs smooth at lower speeds. Tires are new, there was a vibration at braking and I replaced the front rotors so I fixed that now what?