Constant reboots. Did I break second PSU in two weeks?

Question

I have had some bad luck with my newly built PC which I have only had for two weeks. It worked fine for the first week. Then, at some random moment (it was not under any heavy load) it rebooted, and from that point it kept rebooting every few seconds, usually before Windows finished loading, sometimes already on the login screen.

By different means I ruled out a software problem and issues with most components (described below). Having narrowed options down to the motherboard, the CPU, and the PSU I got convinced the issue must be with the motherboard, so I replaced it (for an identical model): it made no difference. So I replaced the PSU next (an altogether different brand). Bingo! The PC booted up fine and worked perfectly, I managed to game on it for several hours. But only for 2 days: then the same thing happened: a random reboot that keeps happening every few seconds now.

I do not really know what to think of it:

1. Was this indeed the PSU that was broken, and did the second PSU break in the exact same way in a matter of days?
2. Was this not a PSU problem to begin with?

Both options require me to ask some questions:

1. If the second PSU broke in a few days, then why? There surely must be something, either in the PC or externally to it, that keeps destroying PSUs. Could this be an issue with the electric circuits of the house? But the power outlet is wired correctly (tested with a little gizmo), the PC is plugged to a surge protected extension lead, and none of my other electronics ever had an issue. Even if there was something wrong with power, how likely is it it would end up causing the same symptoms, in PSUs of different brands (Corsair and EVGA)?
2. If the problem is not the PSU, then why did replacing the PSU make a difference for the two days when everything was fine? Could this really be an issue with the CPU, the only component which I did not rule out? Or did I miss something earlier?

Both options sound equally illogical to me at this point... I was hoping someone here could point me in the right direction.

Ruling out components

A detailed account what I've tried so far, for completeness:

1. Software and SSD: the issue occurs even when booting from a USB stick containing Ubuntu or the Windows installer on it. Even when the SSD is physically removed.
2. Memory: I do not have a replacement memory to test with, however, the problem persists even if any one of the two sticks is removed. I also ran MemTest86 for 10h, and it did not find any issues (during normal operation the issue manifests itself in a matter of seconds)
3. Graphics card: I bought a cheap, second-hand card to test with: same issue.
4. Overheating: I can view all temperature readings in the BIOS, they are fine. Reboots occur straight away after turning the PC on the first time in a day.
5. Motherboard: I replaced it: unnecessarily, as it turned out, the same issue happened immediately with the replacement.
6. CPU: Not completely ruled out but unlikely to be an issue. MemTest86 ran on all of its cores, so that's some indication none is broken. From what I gather, the PC would not turn on at all with a fried CPU.

One reason why I did not suspect the PSU at first was the fact that reboots did not occur when running MemTest86 or a couple other utilities I found on Ultimate Boot CD. If the PSU was an issue, surely it should not make a difference what software the PC was running? Even running some CPU stress test, which I suppose should require significant power from the PSU, did not cause a reboot. For a similar reason, at some point I even thought the graphics card was a likely culprit: what all the software that triggers the reboots (Windows, Ubuntu USB stick, Windows installation USB stick) have in common is that they have a GUI, while the test applications run in a simple character output. But replacing the card for the one I got for £8 on Ebay proved it was not a GPU problem.

Can anyone make any sense of it?

UPDATE 8 Nov 2019: I replaced the PSU again, but the 3rd PSU had no magic powers and did not fix my PC, even temporarily: I got the same issue straight away. Before I go mad and start replacing the CPU I guess I will have to pay for a professional repair service. I clearly can't diagnose the problem accurately if I only own a single PC and no spare parts.

Answer 1

Just for a sense of closure, I will note here that the PSU was not the problem. After diagnosis by a professional repair service, the CPU was returned to AMD on warranty. The PC has been working fine for several weeks now with the replacement CPU.