I would run GPU-z and log your gpu temps and something to monitor the CPU temps. If neither of those are overheating, it's something else, but heat can usually cause it. You need to log and monitor the temps in the games tht do it though.
For a random reboot I would suspect the power supply or the Ram. Ram being the easy one to test pull out one stick and try again then flip them around if still happens. If it happens with both then power supply time..
Most of the time ram and temp issues will cause a blue screen that is why I doubt it is either of these. With the power supply if the droop gets to low it will just restart not giving the OS enough time to even think about a blue screen.
its not a restart, shut down and then turn on and enter to black screen ( your computer not shutdown properly with option, safe mode and normaly ), my builder already clean and flip the ram