Great well at least you know what the issue is, these cards are voltage locked as far as I am aware, I have a program to alter the bios and tried initially to up the voltage to 1.25 as its set to 1.2 in bios but no cigar, so I changed the max clock in the bios to 1050 and all is well. (give me a shout if you want links and instructions)
I have a feeling that my card was returned due to this issue and having read online it seems a common issue with these gigabyte cards (they just have not done enough testing with the factory overclock) I would either return the card for another brand or be happy with the slight downclock. I only paid £200 for mine a month ago and its only for my other PC so was happy with my fix, Your mileage may vary
Defo not an AMD issue though and my asus 290 DCII is rock steady

I wont be buying gigabyte again though due to the voltage locking and the issues with this card and also my old gtx670 which would not run correctly with pcie 3.0