Those symptoms are pretty specific. What you have probably done is set up two different pieces of code that each set the motor to either run or stop. What you need to is to connect them so that the motor is only being set in exactly one place.
If you're using Select functions, delete the Motor Set after the first one and wire its output instead to where you used to have a 0 going into the second one.
If you're using Case blocks...use Select functions instead. Look
here for an example of how to do it.