I looked up what that unit is so I'm not uber familiar and this is before coffee, however, check your effects levels, possibly delays or echos are building up on an individual channel and adding to your total output making it clip. Even if you don't have the effects on output, they can work in the background and clip internally. Is the output sound affected? Magnetic interference is another possibility if you are running it close to pickups, recorders and other high power supplies. Trim levels set too high? I don't know your actual stage setup but these are a couple things I have experienced through the years. Ahh, here's coffee and a drink to you finding your problem.