I am having the same problem with a Gigabyte Z68Xp-ud3 motherboard and a GTX 670. On the x16 slot I get no display, if I swap the card to the x8 slot it works.
It's not the x16 slot as my HD 7850 works fine in it, and it's not the GTX 670 as that works fine in other computers x16 slot.
I have tried everything I can think of but the problem persists. I have searched the net and found many others having the same type of problem, and it would seem that there is some incompatibility with GB motherboards and some pcie video cards.
Question is.....will it ever get addressed?