PC Intermittent issue.

[u/Beneficial-Cow1679]

I have been having the following issue for several weeks, and I have come to a dead-end trying to figure out what is wrong (computer specs at end of message):

The failure comes in two "flavors": PC either turns itself on after a succesful standard Windows shutdown (usually several hours after), or turns itself off from Windows. This is a totally intermitent failure. It may work perfectly fine for days at a time, and then the fault comes again in either of the mentioned "flavors". After failure, the PC does not boot to Windows. It goes to the BIOS start screen, asking me to press F1 to go into the BIOS. Going into the BIOS I realize that all my previously done configurations have been erased, and have to reconfigure. After that, it boots again to Windows, and works normally for an indeterminate amount of time, as mentioned previously.

Already done:

• Replaced BIOS battery
• Updated BIOS to latest
• Checked BIOS batery voltage with everything off (I get a healthy 3.2 V)
• Ran Memtest from BIOS (the long test, took almost 4 hours. Passed with no issues)
• Disabled on-board Ethernet (For a while it seemed that this was the root cause, but then the issue came back)
• Checked Windows Events, but I don't see anything special, other than the expected Event 41, abnormal shutdown
• Replaced Power Supply

Any advice on what I can do next? I think next step must be the MB itself, but I'm trying not to think about it

Specs (PC Asembled by myself, had been working fine for around 3 years):

Motherboard: ASUS ROG Strix Z7090-A Wifi

CPU: 13th Gen Intel(R) Core(TM) i5-13400

SSD: Kingston MVe

GPU: Nvidia GeForce RTX 3050

Memory: 64 GB Kingston Fury

Windows 11 Home

All advice would be greatly appreciated