download and run OCCT PSU Stress test and see if it shut down while the test is running.
Keep a close look at the Temperatures and PSU Outputs for abnormal readings while the test is running.
I suggest having a monitoring program running showing Temps, Voltages, Frequencies, etc. Then run a game and look at the Monitoring program and see if you can see anything abnormal in Temps or Voltages.
PSU outputs should have a +/-5% differences from it reference PSU voltage.