Question power supply or motherboard

Jun 28, 2020
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recently when i tried to play games my pc would restart as soon as the games started to load. this PC is at least 3+ years old. i noticed the fan wasn't moving in the psu, sent it off for warranty. while i was waiting i used another psu same wattage diff brand.. everything worked great.. got a new replacement from the warranty and my PC is doing the same thing again.. however, while i was waiting on the warranty i put the psu from my other PC in and everything worked fine. and if i use the new psu in the other pc it works just fine when playing games. so why is this psu keeping 1 computer from playing games but the other works just fine and why all of the sudden after 3+ years. would this be more of a motherboard issue perhaps?
and before i sent the psu off for warranty i tried clean installing windows, turning off oc settings, setting mb back to defaults, basically tried to make everything like i just built the pc