Computer randomly restarting for no valid reason

I have a 8 year old custom build gaming PC with Windows 10 that up until about a month ago was working with no problems, albeit it was running quite slow. Out of nowhere my computer started restarting with no error message out of the blue and no warning as to it happening.

I took the computer into my local repair shop and explained the issue. They ran numerous tests on it and found that Windows was corrupt as well as several unnecessary programs conflicting with each other so they installed a new SSD onto the computer with an new install of Windows on it. They also ran diagnostic tests and found nothing else wrong.

I took it back and within hours the computer started randomly restarting again so I took it back to the repair shop. They ran and overnight stress test on it for 18 hours and it passed without restarting once. They switched off automatic restarts which they thought was the main problem.

I brought it back home and AGAIN the issue was still not resolved, so went back again and this time they suggested trying a new power lead and plugging it directly into a wall as it could be a power-issue. I have tried that and the problem is STILL persisting.

I am at my whit's end here. The computer shop never had it restart on them when playing multiple different games on it for hours, but for me it's restarting when just doing basics like Office and Youtube. It's a complete mystery what is causing this problem and I have no idea what else can be done.

What other suggestions do you all have that I can try to resolve this?

How about different power socket? I think your house have electricity spikes in your house or your power socket can't deliver the power required to play games - if it's working fine for hours in the tech shop and you've seen it in your own eyes.

@gerygo: that could be the issue. This morning when I tried booting up the computer it restarted after about 5 mins, just idle on the desktop and when it restarted it did so again three times until I got the "Windows has failed to start" window. This has now happened multiple times. I now have it plugged in to a socket downstairs in my lounge connected to my TV and I will see if I have the same issues when plugged into a different power socket.

@Comic_Capers: Update: I tried plugging it into different sockets around the house and the issue is still happening. I also have 40 critical events in my event viewer, source "Kernel-Power". Now what else could I try?

@Comic_Capers: It doesn't add up to anything, if nothing wrong in the repair shop / tech lab while you're having problems at your house, the only logical thinking is you buy a brand new PC or go to some normal tech lab for them to identify your problem and fix it.

Kernal Power doesn't say anything, your PC reports it's been shut down unexpectedly without you demand for it.