It doesn't have to be your video cards that over heat to force your system to shut down. What are your CPU temps?
Does your system crash with just one card in?
Lastly, does your system crash when cards are not overclocked?
One thing about overclocking, just because the slider goes to 950 doesn't mean it can actually do 950. The first step to determine what's wrong here is to set the cards back to factory clocks and see what happens.
My CPU temps are fine, I guess, ~49°C.
When the cards are not overclocked, the system doesn't crash. I suppose I have to test and see what my cards overclock limit is.
Overclock in small increments and test each increase until you are sure it's stable. Once instability sets in just go back to last good overclock and you are set.