I have the Corsair Professional Series Gold AX850. Been using it since January 2012 (so 8 years of usage). I also placed an order for an EVGA SuperNOVA 850 P2 yesterday. Waiting for it to arrive. I probably should have asked this question before ordering a new power supply, but oh well.
 
Yesterday, I was playing Apex Legends. About 30-35 minutes in, my PC just turned off. After about 3-4 seconds, it turned itself back on again. My temps are fine and I cleaned/dusted my PC just a few weeks ago. This is the first time something like this has happened. Then I played another 40 minute round of Apex Legends. Everything was fine. After quitting the game and getting back to the desktop, 3-4 seconds later the PC just turned off. This time the PC did not power back on automatically like it did the first time. I tried pressing the power button on the case multiple times and still nothing would happen. I then turned off the main power switch on the power supply, waited 5 seconds, and turned the switch back on and tried booting up the PC. This worked.
 
I got the following events in Event Viewer regarding these two unexpected shut downs. I've been told these events are usually related to the power supply:
 

 
As for checking my power supply voltages, I don't have a multimeter so I used HWiNFO64. My 3930K is overclocked to 4.2 GHz (24/7) at a fixed Vcore of 1.235 volts. This overclock is not new and it has been in place for quite some time now.
 
At idle:
 

 
At load (after 1 run of IntelBurnTest at the "Maximum" stress level):
 

 
By the way, during that run of IntelBurnTest, for the first time my screen flashed off and then back on. It happened once. IntelBurnTest continued and said I passed the test though.
 
Any thoughts?

Those events in Event Viewer just mean that your PC wasn't shut down properly, for whatever reason.
 
Could be your PSU is failing..Unfortunately, sometimes the only way to know if an electronic part is failing is to swap it out with a known good part.
 
Some of the PSU's built-in protections will "latch off" and won't reset unless the PSU is switched off or unplugged, which would explain why it worked again after you switched it off.
 
See how the new PSU works out.
 
Good luck.

Looks like the warranty was seven years so you're just out of warranty. 
 

 
Thanks for the feedback.

Sprkd1
 
Received my new power supply few hours ago. Just got done installing it. Ran IntelBurnTest and the power supply voltages seem far more stable (i.e. less fluctuations and stay around where they should be). Also played around 30 minutes of Apex Legends and all is good. I don't know if I am being retarded or what but my PC seems far more "snappier" on the desktop and even while browsing the Internet. Just tried loading www.apple.com and it loaded instantly where as before it would take at least a second or two to complete loading/rendering. Good stuff so far.

Thought I would just mention one small nitpick I had regarding the cables that came with my new SuperNOVA 850 P2. I have the EVGA GeForce GTX 1080 Ti FTW3 GAMING graphics card and it requires two 8-pin power cables. The PCIe power cables that come with the 850 P2 are two 1 x 8-pin and two 1 x 8-pin + 6-pin. It did not come with any 8-pin + 8-pin power cables. I ended up having to use two 1 x 8-pin cables. Other than that, everything's great.

Okay, didn't know that. Thanks.