R9280x consumes 350W so 290 needs more to work and hash properly.
sorry, just not true, I have one... 
people should start to look at the spec. rather than making assumption
i'm using Asus R9280x-DC2T-3GD5 gpuclock:1080 gpumemclock:1600 using catalyst 14.6RC2. so MY card consumes 350W, dunno about yours. 24A at 12Vrail
That card btw had ROI in 3 months and since then all is profit. Was the days were doge had met moon! Epic days!!