cpu freezes after few hrs of mining. r9 280x saphire x 4

[8-bit]

i have a 4 gpu rig on Ubuntu 13.10 with cg miner 3.7.2 and 13.11 cat drivers

i can get 680kh p s , although + or - 30kh till it levels off.

but it freezes in a way that scramble the graphics and i have to power down manually. i can mine 6 hours and it will happen or i can mine just an hour.

-I 13 -g 2 -w 256 --thread-concurrency 8192 --gpu-powertune -20 --temp-cutoff 80 --temp-overheat 75 --temp-target 70 --gpu-fan 15-85 --auto-fan --gpu-engine 1000 --gpu-memclock 1500

thanks for help.