I own a Focusrite Saffire audio interface, which I have used successfully for a couple years on my Windows XP desktop PC. But when I use it in the same way (as the main audio out for system and software specific purposes), I very frequently, but randomly, have the audio drop out completely. It appears as though there IS signal, looking at the audio device window, but nothing is coming out. The only way to restore sound is by disabling the device, then disabling the firewire port, re-enabling the port, and finally re-enabling the device. Needless to say, this is less than ideal.
I've been told that it might be a firewire cable issue - does that sound like a sensible source of the issue? I would assume that if the cable was "cutting out", it would show the device as "unpluged", not plugged in and apparently functioning...