RMA it, it's broken.
Had the same Issue with two Asus 7970 Matrix Platinum i purchased after another.
It is broken VRAM , here is an site where you can see where the problem is http://www.playtool.com/pages/artifacts/artifacts.html
The post i made when i faced this issue, it's german but i have linked a few pictures/videos with the same issue.
Before i had a Asus HD6950 DCII without issues.
Now i have a R9 290 and it's totally fine.
So it's not a driver issue.