Didn't have any issues with the 6XX series and black screens. If you can return the sound card, that would be the best interest at this point. As it wasn't that great of a sound card to begin with.

ozzydog153 wrote:
anyone here running a gforce 6 series card with the asus phoebus. have been getting black screens with system lockups after about an hour of gameplay using the latest 314.07 nvidia drivers. if i have dolby enabled for the phoebus no lockups but disable dolby and the system crashes to black screen. only happens with the latest nvidia drivers if i go back to 310.70 phoebus and 660 sli behave 🙂

Make sure there is sufficient airflow around the soundcard. I found running 3 X 570s in SLI with the card sandwiched in between in my Lian-Li case caused higher bitrates to drop out on the optical output - the airflow in my case is poor and the Phoebus gets damn hot. Card was fine when there was enough airflow. Might be unrelated to your case, but the fact your failures are after an hour of gameplay suggests it could be temp related.

-Raja

i think this patch fixed all my black screen crashes looks like it was a tdr problem

patch
http://support.microsoft.com/kb/2665946