Pc randomly restarts occasionally while playing vidya

[Mango_]

My pc never used to do this in the past. This is a recent problem. Every now and again, my computer fully restarts. It usually does it while gaming, but sometimes just randomly when im watching youtube. Because my pc never used to do this in the past, i dont think it could be hardware, but all the signs point towards that.

Specs:
Intel i5 6600K CPU @3.5 Ghz
8 GB ram
GTX 970
550V PSU

 

[sizzling]

Make and model of psu?