I talked about the issue more in detail in THIS
Took me quite a stressful week to find out the real issue.
Initially I obviously thought about SW/HW issues with the graphic card. Then after trying all sort of drivers and after seeing the videocard worked on another PC, I kinda ruled the possibility out and tested everything else.
In the end it was exactely the videocard, but the issue appears only when it's working in PCIE3.0 16x, works perfectly in all other modes.
My Motherboard only has two settings for PCIE ports: "Auto" and "Gen1" (PCIE1.1)
The latest released GPU bios, F2, doesn't solve the issue. Tbh I don't even know if this is the sort of issue that can be solved with bios updates.
Why don't you check your motherboard forums though? There might be other ways (new bios? or software stuff?) to "limit" your PCIE to 2.0.
I "solved" the issue sending my card to RMA asking for a refund (directly at the reseller) and buying a new card.
From what I can say this issue affects only a very small minority of Windforce 670 and an even smaller minority of 680. There seems to be quite a few in the USA but much less in Europe.
It has to be said that there could be more "defective" cards around, but people never noticed because they are using them in PCIE2.0.
Sounds very strange to me that Gigabyte still hasn't acknowledged this issue yet.