Gaming Computer Restarts when gaming

[mahadonation]

Custom-built gaming computer (i7 4790k, GTX 970, 16GB RAM, Corsair 750W PSU, Asus Monitor 144Hz)

restarts when playing modern games such as Fortnite, CS:GO Assassin's Creed Black Flag, Tekken 7 etc

The whole rig is 2 1/2 years old
I cleaned the dust out of it and the PSU
I tested the PSU with a multimeter, voltages all seemed fine
Bought new RAM sticks
Reseated the GPU Reinstalled windows Scanned all drives for Viruses
People online with the same problem say it is a failing PSU.
It restarts after a random time 10 min, 1 min, 1 hr while gaming

 

[vapour]

1. Unplug gpu and try gaming with IGPU only, see if it fails
2. Borrow a good psu and test. What psu model is it? Voltage is not telling anything, it is how much the current it can provide.