Sorry for a firing squad of questions!

Does anything change on the display when the sound goes out indicating a loss or change in coding of the signal? Is it mid song/movie? Both channels at the same time? Any static or pops or a clean cut? When the sounds comes back does it quickly ramp up from silence to volume or cut back at original volume?

You mentioned optical, do you have the ability to feed a digital signal over coax to compare?

Regarding stereo only - have you run through the reset procedure and double checked that a stereo mode isn't configured or forced?