I volunteer at a school that has a DMX system that was installed in 1992. When yo power up the ligghtboard & raise the master to full, the lights on "phase A" will all flicker together. The lights on phases B & C do not. If I take the grand master (or Submaster A) down to 90% the flickering stops. Average load per channel is 500-600 watts. Phases B & C never flicker-no matter what level master is set at. There are 2 separate input ports (one in rear of auditorium, other on stage). I have tried plugging the board into each input--flickering occurs with either input. I tried swapping out the board with another one. Same EXACT results--flicker on phase A if master is at full; no flicker if less than/equal to 90%. Ideas??