Keep in mind that
Keep in mind that with another box in play you wonder where the issue is. The usual drop out I see is simply dust in those optical ports or some smudge on the cable end. But you don't use that so it does not apply.
We are seeing dropouts at the source at times so poke around the cable box and see if it can be set to stereo since the TV is just that and anything more can cause odd issues that we can avoid.