I have a 520W psu and my cpu and gpu together require around 300W:
Checked in GPU-Z and it says 16x
Checked memory with windows memory diagnostic and no error was found
Turned on debug mode in nvidia settings and played gw2. At the end of a story mission, when I left the instance, my computer screen turned black and then after a few seconds it restarted automatically.
I had GPU-Z running and logging in the background, pastebin etc wont let me paste it because it's too much so I uploaded the txt file: https://www.file-upload.net/en/download-13134009/GPU-ZSensorLog.txt.html
I will unplug and replug my gpu tomorrow, when I'm at my computer again