How long is the Cardas COAX? If it's too short, you could be experiencing signal reflections back to the spdif transmitter. I have also experienced differences in quality of toslink cables. The cheaper ones will have a single plastic mono-filament. The better ones will have a high count glass fiber medium. Look at the Lifatec cable, or the upper end Wire World cable.
For the COAX, try a 6 foot Blue Jeans cable Beldon 1694A. Or move up to something like a DH Labs D-750 cable.