I would look in Event Viewer to see if there is an entry in the seconds before it restarts to see if there is any clue as to why it restarted. There may be nothing as Windows may not have any time to create a log entry before the reboot/shutdown happens. But it is still worth checking.
This still sounds like it could be a heat issue. It should be noted that other components besides the CPU and GPU are sensitive to heat. So while 60 and 70°C under stress are fine, RAM, the chipset and VRMs (voltage regulator modules) and other devices may be over heating. You need to make sure the interior is clean of heat trapping dust. If it is, then consider opening up the side panel and blasting a desk fan in there during your extended gaming and see what happens.
I always want to ensure I am supplying good, clean stable power while troubleshooting potential hardware issues. So if me, I would swap in a spare PSU too.
If you do need a new PSU, note while Corsair used to have a great reputation across their entire line, in recent years that has faltered as they changed OEM suppliers. Their top-of-line series are still some of the best ever made, but that is not so in their low-end, entry level lines. As seen here, the
CX series is next to the bottom. I personally have been using EVGA in all my builds and like Seasonic too. Of course, until Man can create perfection 100% of the time, even the best of the best can make a unit that fails prematurely.
Are drivers, especially graphics, current? Is it always the same game?