It sounds like either a lead problem, or a mixer one. First swap the leads completely on both decks, and see if the same channel cuts out, if it does, then its a mixer problem. If its only on one channel, and it is the mixer, then its probably something to do with the up fader, the line switch, or the connectors on the back of the mixer. When it happens, move stuff to see if you can sort it, ie... Wiggle the faders, the line switches, the sockets on the back, and see if you can pin point the problem...