Assuming your drivers are up to date, your system is fully restarting or shutting down, and it is an issue with your hardware or its setup. In short, the hardware is getting itself in a fuzzy state, which is causing it to reset the system. It could be anything from power supply, a loose connector, or a bad capacitor to something software tunable like overclocking or boost settings. I had a similar issue a few years ago, and it turned out that the boost software for my graphics card and the RGB software for my motherboard would sometimes go head to head with each other and causing the entire system to crash randomly when gaming.