[Alan Anderson]

I'm going to guess that you're using a pair of buttons to control the motor. You probably have an if/then for each button, where the first chooses either forward or off, and the second chooses either reverse or off. If you press the second button, you get reverse power to the motor. But if you press the first button, you get forward power just for an instant before the second if/then shuts it off again.

If that's what your code is doing, you need to fix it to turn off the motor only if neither button is pressed.