HELP! MINING RIG SHUTS DOWN AFTER 1 HOUR

[colinacameron]

Hey,
I'm running 2x sapphire radeon 290x with 8GB ram, i7 4770k, and Corsair TX750W PSU, I am mining wdc and after an hour of mining the rig shuts down. I have the shut down temp at 99 degrees.
Settings: Sapphire 290X: -i 20 -g 1 -w 512 --thread-conc 32765 --lookup-gap 2 --gpu-memclock 1500 --gpu-engine 920 --gpu-powertune 20

879 Kh/s 92 degrees 57% fanspeed

CPU not overclocked. Max power usage per card is 300W. I really need help with this.

Thanks.