I had this problem a few weeks ago, whenever anything graphically intensive was used my computer would turn itself off, turned out my PSU wasn't able to cope with the power needed to fully run my graphics card as it (my PSU) was getting old. Even though you have stated your parts are only about 1.5 years old that doesn't mean the parts that make up your PSU are fine, you could have been sold one with slightly bad parts which has caused it to expend its life span much sooner. IF you have a friend with a 700W PSU you can borrow off them to test it you can see it's most likely a PSU problem.

Also when people mention about temps they aren't just talking about GPU temps, they are talking about CPU temps as when the CPU gets too hot all systems turn themselves off to prevent damage to the CPU.