I offer a 30 darkcoin Bounty to someone abble to fix my problem with my R9 290.
Summary: my cards started at 2.4MH/S and after 10 minutes they loose 10% and are stable at 2.1MH/S
This happened on all my cards, on all motherboards, under BAMT.
I noticed all the cards have the drop on the exact same second (on one motherboard of course).
I have 2 other cards on windows 8.1 that are stable at 2.5MH and from time to tim (this morning) I have the same drop (watercooling cards)
When the drop happened, the only way to recover the lost hash is to shutdown the machine (a mine restart did not do the job: I don't have higher hashrate)
If I put another card on the same rig, after 10 min, all the 290 will have a drop, and the other card (7950/270/7970) will not have any loss.
If I use thoses cards to mine Scrypt coins, no drop.
Further developpement:
I downloaded the BAMT version from Bajawah with sph-miner. No change.
I bought a 860W platinium PSU. I plug 2 290 ands the mother board on that psu, and the third 290 and the 270 on another 750w psu: about 10 minutes laters, on the exact same second, the 3 290 (on 2 different psu) loose 10% hashrate.
So, I think it is not a psu problem. (bad purchase)
The 30 darkcoin bounty is still valid.