As Sunius stated, you need to make sure to connect it to the dual-link output on the card. I believe the DirectCU II 6950 has 2 DVI ports, only 1 of which is dual-link. Make sure you connect it to the right one.
Also, DVI-D doesn't mean that it's dual-link. There are dual-link DVI-I cables and single-link DVI-D cables. Make sure yours is dual-link. http://en.wikipedia.org/wiki/Digital_Visual_Interface