Massive->Regular->Occasional Autorebooting?

rebooting due to overheating would be your cpu not your graphics card, I advise you to reboot press delete to enter bios screen, there should be some hardware monitor tab which will show the temp of your cpu, from a cold start with no load it should sit between 35-45 degrees centigrade after full load with a decent cooler it should not be higher than 55 degrees centigrade, also what stands out the most with you PC spec is the power supply the wattage is very low, recommend 500W as this could also be causing random reboots