Josh, the problem is with the structure of your if statements. If one button is pressed and not the other, the motor will first be set to 63, then will be immediately set to 0. Or it will first be set to 0, then will be immediately set to -63. A better way to do it is:
Now this will work this way:
Neither button pressed = motor set to 0
Up button pressed, Down button not pressed = motor set to 63
Up button pressed, and Down button pressed = motor set to 63
Up button not pressed, Down button pressed = motor set to -63
Shouldn't jitter any more.
cu,
Gary