It sounds like your implementation is causing multiple (different) values to be set to the motor during one iteration of your code.
You'll also want to determine which button you want to have priority (if both are pressed, what speed do you want the motor to spin at?). Then, you can either use a
case structure or two
select nodes to choose the motor speed, which you would then wire into a motor set VI. Try and write the code so that there's only one instance of the Motor Set VI controlling the motor.