Gigabyte GA-Z77X-D3H MB problems

I had exactly the same problem and figured out it is was the cable not the motherboard! The spec for Displayport cables directs that pin 20 (power) should not be connected. However, many (cheaper) cables don't adhere to this. Some devices (esp Dell monitors and Gigabyte motherboards) fail completely in this situation - I assume other devices are somehow better configured to handle unexpected power being received, though it's hard to blame Gigabyte.

My solution was to break pin 20 in the cable connector (it's the bottom right pin looking in to the cable, with the 'U'-shaped grooves pointing downwards). A branded cable would also work. Maybe Gigabyte will also eventually update their BIOS to handle this (as of F14 it is not working), though it could also be a hardware problem that can never be fixed in BIOS. Also the engineers may never reproduce the issue in the lab if they are using 'proper' cables...