I sorted it out the problem of my rig of  2x 280x toxic sapphire,if i run 2 gpus on 850w psu, my system shut down after a few seconds and i need to switch on and off a few times that i could turn it on,but if i run 1 gpu and -I 13 then everything ok and almost stable by 807 kh/s 73 temperature,im gonna get better gold psu sorry for english