Okay, I use a Presonus Firebox mostly for listening via headphones.

What is really annoying is that the sound will periodically cut off...
I've changed clock source from "Device" to " Mac", that seems to help a little but I know that it not the correct setup.

The device is getting the power from the Firewire connection only but it should not be a problem as this is Mac Pro.

Since this is an intermittent problem, it is rather hard to predict but it does happen every time I do listen to music.

And yes, it does happen when I record, so needless to say, this device is useless for recording.

Any hints on troubleshooting?

For those that might read this, the problem was the Sonnet FireWire + USB 2.0 Card Since I needed a PCI-E card to plug in my FW400 device. The card seems to give or have electrical interference.

Annoying to say the less and I can't really return the card at this late date....