this maybe a dirty fader causing this depending on which type is fitted, if it is an optical fader then there is an internal fault and it will need to be returned and fixed however if it is a carbon fader then the likelihood is that it is just a build up of carbon deposits and dust so when you pass the fader over it is cuts in and out, a basic switch cleaning lubricant will resolve this by simply squirting a small amount in each end and working the fader back and forth several times and it should clear.