This is most likely a hardware related issue caused by the DAG being over 2GB now. This causes something like the DAG now being spread over multiple banks of memory, slowing down access. The bigger the DAG gets, the more often the memory controller has to switch banks, the slower it responds. In reality it probably is a little bit different/nuanced, but I guess it comes down to the same thing. The most important question is; how much are the 290/390's affected? Because when they have it too, the majority is in the same sinking ship, so it's not that big of a deal. Good news for Pascal owners though, they should be fine as a similar issue was fixed when that hit the market last year.