Assuming optical SPDIF, you might want to confirm the Word Clock settings are configured correctly. In all digital audio signals, there has to be a signal that provides a common understanding of where the bit words begin and end, this is called word clock. SPDIF signals include a word clock signal along the optical wire, and each device must correctly understand who is the source for clock and who is the slave. Usually "hub" devices are slaves by default and "playback/generator" devices like disc players or media decks are source. If either of your devices has a manual setup for this (most consumer devices don't most pro devices do) ensure that one is the source and the other is the slave. If both are set to source, you will get varying levels of static along with the signal. Sometimes the static is constant, sometimes rhythmic, sometimes blip, sometimes not present.
Hope thats it. Otherwise its a degenerating part or something. Usually optical cables are pretty foolproof, I would suspect it last.
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