It's seem happen to me.

I got HIS R 290 (non X) last monday. It has 2560 shader, viewed by GPUz.
I spent last 3 days for tweaking and setting.
1st day i got 885 Khs, with --thread concurrency 32765 --gpu-engine 975 --gpu-memclock 1500 -I 20. It's rock and stable.
This card got 99,000 shared, WITHOUT Reject or stale. It has 70 C, and Fan only 65%.

You know in my country ( Indonesia ), It's pretty warm here. Ambient always @ 33-36 C.
My 7950 and R 280x always rise up to 78 C,  tough I 've undervolted them, 7950@0.965v, R280x @1.146v.

But the problem was occur. In my pool, hashrate goes up and down like a crazy. The lowest = 319 Kh, The Highest = 1.310 Mh.
Wide different, isn't it ?

Try to powertune +20, +30. there was no effect.
The effect only it will rise my electricity for 70w for powertune +30.
And temperature will rise 80 C <.

2nd day until now,
I try lower intensity, lowering Thread concurrency, trying downclock, update BIOS with the other brand. (ASUS, Sapphire, etc),
but it still no good side, rather than lowered hashrate @ 770 Khash.   

This R9 290 nice card, for performance and wattage.
But for pool hashrate instability, it's still became our research.

Thanks for this great forum.