PC shuts down randomly, company could not find the issue

[TheFatBaconGuy]

My computer is shutting off randomly. (I build the PC myself with all the components from one store, I reused a hard drive from my former pc, the other parts are new) It is a complete, no warning shut down. No BSOD, no visual clues at all. The computer then will power itself on after a short time. It will shut down in low performance and high performance situations. It happens mostly while gaming. It happens in the middle of a game and always happens exactly when a League of Legends game ends. At the exact moment the match ends my computer shuts down.
I've checked the CPU and GPU temp logs and they are fine, my PC isn't overheating.
I have a Coolmaster power supply that should be capable to provide by far enough power for my PC. I even formated the PC twice in case it was a virus.
I even brought the pc to the store where I bought the components and the first time they looked in to it they tested it but it didn't shut down, but when I played at home it did shut down. So I brought it to them a second time and when they tested it it did shut down. So they where repairing it and after 2 weaks they gave it back.
When I played at home it shut down again, so I brought it back to them the tird time. After 2 weeks they called and said that they didn't found any problem, they did a torture test for 70 hours and it didn't shut down. I did the torture test at home a lot, in the beginning it would shut down immidiatly, but after a while the PC didn't shut down anymore with the torture test.
The shutdowns are so weird, I really don't know what I should do next to find the problem

I hope someone can help me

 

[salerhino]

Well, I hope that they in the store actually replaced 1 by 1 component.
And we can tel you that the problem can range from faulty PSU-MOBO to something funny like bad wiring or contacts with on and reset button on case.

 

[panathas]

I think you should test your PC with another PSU. Borrow one from a friend if you don't have a spare one. Just make sure that the one you choose is in good condition. Sometimes PSUs behave differently in various environmental and electrical conditions, meaning that the PSU may behave differently when it is working inside you room/house and when it's in the store. Especially if the quality of the PSU is questionable. What's the exact model number of the PSU? Coolermaster has some good PSUs and some that aren't so good.

 

[TheFatBaconGuy]

Well, I hope that they in the store actually replaced 1 by 1 component.
And we can tel you that the problem can range from faulty PSU-MOBO to something funny like bad wiring or contacts with on and reset button on case.

What should I do? Replacement and test every component myself?