Left channel drops out on NAD DAC toslink

Bummer, I bought a refurbished NAD C375DAC integrated amp but it's not working properly. I'm trying to play audio through the DAC module, connected to windows7 PC via brand new glass optical cable.

The problem: Intermittently, there's no sound from the left channel. Twice, the left channel has started working only to drop out after 5-10 minutes, and stay dropped out. Can't tell what causes it to start working again. Problem affects speakers and headphones the same. Other sources (CD, Tape, Phono (via Aux)) work just fine.

Any idea what might cause this, other than a busted DAC module? Looks like I need to ship the whole thing back and request a refund. What a shame.
If I had to guess, I'd look at three things:
1. Possible bad cable.
2. Possible loose connection at either end of toslink?
3. Possible conflict w other services running on your PC?

Do you have another digital source; a disk transport,ipod or even your cable box to try? And can you run co-ax or USB out of a digital source to the DAC module?

But I'm just guessing. PCs and DACs are pretty much black boxes to me.
Is the DAC a plug-in module? If so, can you physically remove and re-seat the unit? Maybe just a connection issue from shipping.