Question Monitor loses signal and fans become insanely loud while gaming (once while logging on)

Aug 8, 2019
Hi all, apologies in advance for the long post, but I'm dealing with probably the weirdest issue I've run into in all my computer-fixin' days and could really use an assist.

My brother is having an issue with his desktop I've been trying to troubleshoot for him to now avail. He's been using his machine without any issues for months until suddenly today his monitor just went black while playing Crusader Kings. Music kept playing in the background and we heard the sounds of events triggering so the program wasn't actually frozen, but the monitor was no longer getting any input. Issue kept happening after a clean driver install, and once or twice we observed stranger behavior: as soon as the monitor lost signal his computer started sounding like a hairdryer, extremely loud fans. Couldn't tell if the sound was coming primarily from the GPU or CPU fan, or if it was both. No problem ever happened until we launched a game; things were always fine on the desktop and we were always able to boot in log in. We were even able to run the superposition benchmark to completion without any problems, but within 5 minutes of launching any game the monitor would lose signal, and about every other time the fans would go loud like crazy again as well. In one weird instance he played Mordhau for 15 minutes with no problem only for the issue to happen the moment he launched Crusader Kings, but other than that once instance it would happen within 5-10 minutes of any game. MSI afterburner indicated totally safe tempts at all times (GPU never above 65, CPU never above 60). We appear to have a "Display driver nvlddmkm stopped responding and has successfully recovered" event in the event log for each time the monitor went black, regardless of whether or not the fans ramped up.

Looked into this problem and everything I found online indicated it was probably a power issue. Following steps I found in other posts, we made sure all PSU connections were snug, plugged his desktop and monitor into a new power strip on a different outlet, and set his desktop's power management mode to high performance (it was set to balanced before).
After doing all this he launched the game and was able to play with no issues at all for about 45 minutes (when previously we'd always run into the problem within 5-10, usually closer to 5), and we thought all was well, until suddenly the screen lost input and again all the fans went into maximum overdrive. Again, when this happens we can still hear the music from the game and also sound effects happening, so the game itself hasn't crashed or frozen or anything. Just now I booted his computer back up to run speccy and this time, for the first time ever, the screen went black and fans went into super mode without running a game. Windows login screen came up totally normally, but as soon as he entered his password and pressed enter, it happened. This is the first time it's happened when not running a game.

I'm open to the possibility that his mobo or PSU needs replacing, both are 7+ years old, I just want to try and make sure we're replacing what actually needs to be replaced before we buy anything; I'd hate to rebuild the whole machine on a new mobo and then find out the probelm was the PSU, or replace both and then find out the problem was something else.
We're leaving it off for now to be safe. Specs as far as I can get them are below:

Asus P8Z68V
Gigabyte GTX 1070 G1 Gaming
8GB DDR3 (not sure exact model but it's corsair)
corsair hx650 PSU

Thanks very much in advance for any help.

PS. example from a different user with a very similar problem:
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