Jumping in here to add some info and ask some questions and hopefully help -

What size case are you using, what does the airflow look like inside, and which card is in the top slot? I've never seen GPU temps cause Windows to shutdown but I have to wonder if they aren't raising the ambient temperature to the point where the processor is hitting a thermal threshold and powering down to prevent damage?

I realize this is only my expirience but I was fooling around with the manual fan control in Afterburner and left it at 20% accidentally while mining...

By the time I got back to the computer the temp had spiked to 102C but the computer was still on and still trying to mine away! I used it as an excuse to take the cooler off and reapply the thermal paste but the original stuff was actually still in excellent condition and thankfully I didn't fry the card or any other components. I'm using a 7870 that I haven't messed with OCing until the new thermal paste sets but I can typically hit right around 400 on it alone.