You may or may not be aware, but the expression pedal does not use a traditional potentiometer, but rather uses a system based on infra-red whereby there is s a sender, reflector mounted to the bottom of the expression pedal and a receiver.   Sometimes if you use your X3 Live in extremely bright sunlight or with bright stage lighting at floor level, infra red light can creep in from outside the unit and upset things as you describe.  Most people would hear the effects when the pedal is used for Volume control or Wah. 

However, a ball of fluff from dog hair or similar may also get in the way of the indended infra-red beam if perhaps your unit came from someone who owns a dog or a cat - basically you may need to clean inside and around the underside of the reflector which is attached to the foot pedal