Sounds like it's your PSU failing. That PSU was pretty good in it's day, but 7 years after intorudction (if yours is that old), i'd say its that. The GTX780 is posisbly the same age, it could be that too. But given the description, I'd lean towards PSU.
PSU's degrade over time, and don't supply the same power or efficiency. Your's has had a good run. It would be good practice to upgrade it before you potentially fry any hardware.
i've modded both Skyrim SE and Fallout 4 and this never happened to me. Modded meaning nearing MAX amount of plugins and mods. Check task manager and watch the RAM %, this was a big problem for me with 16 GB of RAM thanks to memory leaks in FO4. The worst that happened with me in the games are crashing to desktop. So it might be a bigger issue such as the PSU failing or what not.
Open up Event Viewer and click on the System tab and check to see what errors are listed and post one of them up, if they pertain to the restarts or not.