PC Shutting off While Gaming

[MrClEaNMON55]

So starting on 4/10/17 I booted up CSGO played a little bit and my machine just shut off and restarted. It did not happen until later that night when I started playing again and it shut off rapidly after booting up CSGO. Today on 4/11/17 I ran the fire mark extreme stress-test and my computer shut off almost immediately after the test started. I think it is probably something to do with a faulty power supply as it is a bit old.
System Specs:
-Antec Gx500
-MSI H97 Motherboard
-16GB Corsair Vengeance Blue DDR3 1600 Mhz (PC3 12800) Desktop Memory
-Intel Core i7-4790k@4ghz (I am a noob who does not know how to OC)
-Corsair Hydro Series H100i GTX High Performance Liquid CPU Cooler
-Nvidia Geforce GTX 980Ti
-Corsair CS Series 750 Watt ATX CS750M
-Samsung 850 EVO 1TB 2.5-Inch SATA III Internal SSD (MZ- 75E1T0B/AM)
I built this machine around September 2014. It started out with some different components like the GPU, CPU heatsink, HDD, Mobo, a little less ram. But the power supply has never been changed so that's why I think it is probably an error within the power supply.

 

[alexoiu]

I suggest testing different or replacing your power supply.