DJM A9 distortion audio master output

HI, 

On a regular basis, but not always, the following happens: if my x-fader is slightly open, the sound is quieter and there is distortion. I hear this on my speakers (master out) and in my recording (rec-out). I don't hear this in my CUE (headphones).

I use cdj 3000 connected with digital SPDIF cable. I have already used new spdif and the problem remains. CDJ's are latest updated. Problem occurs with both USB and LINK via PC.

I have a video if desired.

*I have already had the DJM checked at an official service center and they could not find the fault.