Sapphire 290x (ref) hynix mem.
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"gpu-memclock":"1500", //Finally! i've spent two days to raise memclock to this point. Already wrote some posts with this trouble. And solution is raise it step by step (for me its works), slowly.
Try memclock 1250 (right, lower than stock), you will be surprised. I have Sapphire 290 and with 1250 it's much better than on stock 1300, and a little worse than 1500, but much more stable without voltage manipulation.
Got stable 440kh/s at I:17 and tc:24550.