Before you order any new Ram can you post a Speccy url for us please, details here
8GB modules do seem to be more problematic than the 4GB sticks, I cringe when I see people having computer issues and they only have a single 8GB stick of Ram as you know straight away that if the issue/s are ram related the person is going to be circa £125/$210 out of pocket after the initial loss and replacement costs.
There's the URL. I've put my Kingston HyperX Red ram (2x8gb at 1600) back in as I've already returned the AMD ram. However while returning the AMD ram I did notice they actually sent me the wrong thing. I was just keen to get it unboxed and in the pc unwrapping it so I didn't notice until I removed it that it was "AMD Performance" not "AMD Gamer" so it COULD be that the performance series isn't tested on 990fx boards. But at this point I think i'm going to admit defeat and get 2 sets of 2x4 ram that are on the QVL.