Thanks for your contribution.
My desktop was built around December, 2013. It was working fine prior to switching out the GTX 660. I ended up selling it to a friend back in January. I purchased my GTX 970 in June. Between this time, I had issues trying to use onboard graphics. Every time I tried to run an application the utilizes this, it would freeze my entire session, forcing me to reboot my computer. Once I received my 970, which came from a working unit, I did a fresh install of Windows 10. It was then I started having issues with my computer unexpectedly shutting off. But even with the 970 out of the case, my computer would try to boot up, shuts off, and restart. Sometimes in a loop. It could take up to minutes to even get to the motherboard bootsplash.
The fact that you has some issues with the onboard graphics might be independent of the issues you are having now. If the computer unexpectedly shutting off happened after the Windows 10 fresh install, then it is possible that the Windows install went bad. Have you tried re-installing it again? While my gut still says an issue with the motherboard or PSU, it is possible that maybe the Windows install (or the installation of the motherboard drivers during the fresh install) had some problem. And it would not hurt to try re-installing other than take up some of your time, which may or may not "hurt" depending on your time availability.
I agree, my main two culprits would be either the power supply or the motherboard. Is there any way to check which piece of hardware caused the issue within Windows or the BIOS? (Sorry if this sounds like a stupid question, I'm not this experienced with computers )
Not that I am aware of. Usually the best way to diagnose hardware issues is to isolate that piece of hardware. That either means taking the piece of hardware and trying it on another (preferably know working) computer OR get some known working or new hardware to try in the place of the questionable hardware. So, likely the easiest first option is to get a new PSU to try, whether you actually buy a new one or see if you borrow one from a friend (or another of your computers if you are a TOTAL computer nerd and have multiple computers) would be up to you. I suppose the other option is to test some of the electrical outputs from the various connectors of the PSU, but that is outside my area or expertise (I know enough that sticking my fingers after licking them in an electrical plug is not exactly a good idea
), so you will need someone else to help with that if you want to try to go that route. The advantage of starting with the PSU is that 1) having a backup PSU is not a bad thing to have; and 2) you can generally get a good PSU for less than a mobo many times.
I will add that if the issues you had with the onboard graphics IS related to the now shutting off issue, then that might point a little more to the motherboard. But, it is not clear (at least to me) if those two things are actually related.