Did you happen to listen through Bluetooth headphones?
Release Notes for firmware version 5.0.603548 mention "stuttering in Bluetooth output when using Optical-in or Line-in sources at 192kHz" has been fixed. Honestly, I don't quite see why a CD player should be using 192 kHz sampling rate on its optical output, but updating to the latest firmware (which happens to be version 5.0.604458 as of writing this) should be a good idea, anyway.