When your GPU is pushed to the max your system uses double power if not triple.
try seriously to put some graphic extensive settings down in game and play for a day... and i dont mean turn just shadows 1 notch down..
just try to play with average settings and see what your computer does.. if it still shuts down .. then it's probly time for a better PSU.
And ofcourse if your Graphic card is overclocked.. put those first back to factory settings.