lonewolfraider Hello. Please check your CPU temps, while it's under load, and report the results back in this thread. I'm guessing that there is one of two causes of your rebooting problem: (1) The excess heat from your GPU load combined with insufficient CPU cooler or case airflow is causing your CPU to overheat to the point of thermal throttling and eventual reboot, or (2) Your power supply is malfunctioning, irregardless of wattage load, and a warranty replacement indicated.
question: (1) What is the CPU cooler model that you're using. (2) Approximately how long does it take before a reboot happens?
request: Please take two photos of the inside of your computer case: Have one photo showing the entire motherboard and case fans in the computer, as I'd like to get an idea of airflow, and (2) Take a photo of the power supply label, so that we can see the precise model.
Summary: After you take note of the temps that your CPU reaches under load, then let's test my theory. Please open the side of the case, while the computer is still powered on. Are all the case fans spinning? Is the CPU fan spinning? Now take your computer and place it on it's side, while the computer remains on. The motherboard should be exposed and pointed toward your ceiling. Please try to keep the components cool during this test. Either turn on the AC in the room that contains the computer, or get a floor fan, point it toward the computer, and turn the fan on. Now place the CPU under load again and see if you can reproduce the rebooting.
Btw, it's entirely possible that a new power supply could fail to the point of rebooting the system with minimal load placed on it. I've personally experience such a problem with a high quality PSU. But the cpu overheating is more likely the cause. Please try my suggestions and respond back to this message thread ASAP.