It may be a total power issue. Try just running one videocard and over clocking. I was looking around on google and the min amp output for 1 280 is around 36 amps. You might not have enough power for two, but i dont know.
I ran one card for awhile and OC'd the snot out of it and everythign was fine even with the CPU/RAM OC settings. Add in a second card and bam, system shut down. Both cards are fine individually.