I have built this computer about a month ago and have been having this issue many times. My system will shut it self off and turn back on without warning when playing Rainbow Six Siege, but if I play any other game like Just Cause 3 or Madden then there will be no issues. I also ran the Heaven Benchmark on ultra settings for 30 mins and I have experienced no restarts. Only when I play siege I have this issue. I also sent my computer to Geek Squad 3 times to see if they could fix the issue. They told me that all of my hardware components work and they have no idea what the problem could be. I have checked all of my temps for my CPU and GPU and they show no signs of overheating whatsoever. I have also reinstalled windows and did all the tasks most websites recommend me to do when encountering this problem like disabling fast startup and such. At this point i'm probably just going to sell this computer's parts if there is literally nothing else I can do.
Parts:
Intel i7-9700K
Nvidia RTX 2070 super
Msi Mpg Z390
Corsair CX750M
Corsiar H100i Platnium
Corsair Vengeance LPX 16GB
NZXT H700
Parts:
Intel i7-9700K
Nvidia RTX 2070 super
Msi Mpg Z390
Corsair CX750M
Corsiar H100i Platnium
Corsair Vengeance LPX 16GB
NZXT H700