Hi @John_Roberts ----- Thank you for the report and sharing this observation you have made with us. The feedback and insight are both very appreciated!
Moving forward, to help aide in our understanding of this behavior you are experiencing with the TSR 5790 may I very kindly ask you to please provide the following:
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Please expand on description of your current setup using this link as a guide.
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If you have any other active audio zones besides the TSR 5790 do they display the same behavior?
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While this behavior appears to be intermittent (i.e “occurs several times a day but usually only after several hours…”) are you noticing any patterns here? From the sound of it, it seems like you are noticing this when the radio function kicks in. Does the same occur if you do not allow “radio” start and say just queue up a handful of albums?
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While it may seem a touch elementary, since noticing this behavior have you tried rebooting not only the core machine but also the endpoint as well?
Additionally…
“I am also interested in any suggestions about how to optimize the signal going through this system to speakers.”
I have a posted a few articles below from our Knowledge Base that should help with this 
-Eric