C5D's firmware reads USB voltage even in self-power mode, so the DAC can potentially shut off when there's a cable or power disturbance. The assumption was that the DAC should remain off to conserve power when a USB cable is disconnected. There's some extra logic to avoid dropouts, but this would not help with large variations in USB voltage.
The 2nd batch of C5Ds arrive today and we've updated firmware to force the DAC to remain on even when USB voltage behaves unexpectedly, including complete voltage dropouts. So unless the data drops out, the DAC will not turn off. Please email us if you have trouble again.
Thanks John, glad to hear fast response from you and your team.
I am assuming battery is the main feature that can explain everything relate to power issue and connecting USB to PC is all about data transfer and ground sharing.