I tryed a few drivers, but the fact is that it happened only with Darkcoin mining, so it should put the driver/bios/hardware aside if I'm correct.
I'm pretty sure that there is something in the miner that react baddly with the R9 290.

I just noticed one more thing:
When the loss happened: all my cards (even the non-290) had a huge drop.
After that drop, all the cards climb up again, the other model reach their max hasrate, and the 290 reach only 90% of max hasrate.

Since I'm not really fluent in English here is an example:

Coldreboot
290: 2.5  270: 1.3
10 minutes then a drop in the same second of all the cards in the rigg
290:1.5  270 0.9
Immediatly the hasrate started to recover and a few seconds later:
290: 2.3  270: 1.3  so a loss is visible only on the 290
If I restart the miner, I keep that last hasrate.
If I coldreboot: It starts over.

PS: it happend the same way without overclocking: 290 went from 2.0 to 1.8.