So you're having issues with your computer shutting down? The GPU temperature is fine, I'd say it's possibly more due to an unstable CPU overclock. Going by your sig you have your 2600K at 4.9GHz now, lower it a bit and see if the problem persists. What voltage is your CPU running at Rob?
And your computer just randomly shuts down? Is it when you are putting load on the CPU and/or the GPU or is it just random? Run Prime95 quickly and see what temperatures your CPU gets up to, and then run FurMark and see what temperature your GPU gets up to. Prime95 will stress the CPU and the RAM and FurMark will stress the GPU.
You may want to possibly think about bumping the voltage up on your CPU a bit, 1.125v seems quite low for 4.9, maybe try 1.135 or 1.14 and see if its any better.
Umm if it shuts down in games and GPU intensive things then it is possibly your GPU. Take out your cards and dust them down (just spray them with air), put them back in and see if your temps have dropped.
Could be the power supply I guess but more likely the GPUs. Coming to think about it 81C at idle is pretty high for a 6870. Is that the combined heat of the two cards though?