I'd start with a good cleaning, fresh heatsink compound and be sure the fans work like new. After that I'd get more information to the forum such as:
1. Make, model, AGE of the PSU.
2. The Speccy report from https://www.piriform.com/docs/speccy/using-speccy/publishing-a-speccy-profile-to-the-web
3. Since it's game related, the Userbenchmark.com results too.
Look up DDU as we will most likely disable Windows 10 (if that's in play here) Driver Updates, use DDU then install the video driver you knew to work.
My old PC is suddenly restarting when I'm playing a game. The screen will just cut to black and then it'll start powering up again. I'm not sure what the issue is. Is it just because it's old? I haven't noticed it happening when I'm just browsing the internet or writing reports, only when I try and load a game and start playing it (normally within the first 5 minutes it'll restart). Sometimes the blue screen appears when it restarts again saying "an unexpected error has occurred".
CPU: AMD FX 8120 - eight core
GPU: NVIDIA GeForce GTX 670