Did you get anywhere with this? I would say take the crossfader out and see if that cures the problem. Im not sure on how that mixer is routed, but most of them will just work on the up faders when the xfader is taken out. If that cures it then its your xfader, if it doesnt, then its not. You could then try the same technique with the up fadersm and also switch the upfaders around, if the problem changes onto the other deck, then you know which fader it is. Im not sure tho, it deppends on the mixer.