Try further underclocking that card and see if it shuts down. For the mean time while you wait. Try to do everything you can to reduce power consumption. Any extra HD's remove them, and RAM you don't absolutely need (Anything more than 8gb) remove. Underclock that vid card, and when gaming make sure no other programs are open causing load on the CPU. You can even try running a program like GameBooster to shut down any un needed system resources during game play to further reduce power consumption.