That is a pain, it certainly seems like a power issue rather than an input issue, as it was going on all channels. And you have ruled out the mains in your house as it happened at a gig (unless the gig was in your house?). Does the CQ12T have the same power supply as the CQ18? If so can you try it with that power supply and see if the issue remains, a replacement power supply would be about the cheapest option