[John_Toolbox]

Never had dropouts on a mix system. If the audio files are fine, the problem is somewhere between your D/A and your speakers. I would try sticking something with level meter between each piece of gear between your D/A and speakers and see if it shows the dropouts consistent to what you're hearing. If you find constant levels during the dropout, the next piece of gear in the chain is the problem

Also if you're not using both halves of the ADAT bridge, try moving everything to the B side, and see if the problem persists. I suppose you could also just swap everything between A and B sides if you are using both sections.