I had an issue with my OpticalRendu and I resolved it by moving my ROON Core server so that it was directly attached to my router. Great thing about the Rendu's is that they do not need to be connected to the music server.
I used to have my ROON Core on the slow side of my home network, on the PowerLine adapter part (using in-wall copper lines). The bandwidth on that part of my network was not able to keep up with the hi-res data that I was streaming. The issue was 100% reproducible using George Harrison's All Things Must Pass stream. Specifically a part of a song where George whistles. My music always got garbled a bit during that passage.
When I moved the ROON Core to the faster side of my network, the garbled sound on that passage went away.