PC startup problem related to power supply or motherboard?

Hi all,

I would like to ask your help in diagnosing the cause of a persistent startup issue i am facing.

Symptoms:
For some time now i am having startup problems with a DYI PC build from august 2019.
The system was running normally for two weeks but then suddenly it wouldn't start when performing a cold start.
Only after disconnecting the power cord for couple of minutes (a few seconds wouldn't help) the system would boot again.
Because the system was running without the motherboard grounded via a PC case, I though it might be a discharge issue of some sort.
So I ordered a PC case and in the mean time was disconnecting the power cord after shutting down the system.
Unfortunately, when I build the system into the case the problem did't go away.
Because the problem persisted I started suspecting the CMOS battery and the BIOS.
After flashing the firmware to version F10 (at that time the latest version) and swapping the CMOS battery for a new one, the problems went away for roughly a month but then were back again.
I then again resorted to disconnecting the power cord to make the system perform a cold start. Unfortunately a few weeks back an additional problem arose.
Even tough the system would boot from a cold start (after disconnecting the power cord of course) the system would hang at windows load screen.
First I thought that it was a virus/malware having corrupted my boot manager but it turned out that the BIOS was reset (most likely due to the power issues..). Strangely enough the BIOS reset was raising the CPU voltage to 1.3V making the CPU fan spin very load at boot. However the cause for the system hanging at windows load was caused by the low default memory voltage of 1.2V while my DDR memory requires 1.35V.
Happy, that my windows boot manager was not corrupted I again replace the CMOS battery which again made the system run normally for a few week. (without the need of disconnecting the power cord before performing a cold start).
Of course today the system again hang at the windows load screen and the bios was again reset.
I just replaced the CMOS battery again and flashed the firmware to the latest version F12d.

Apart from the cold start the system seems very stable (no crashes). Also I am not overclocking anything.
In addition to the cold start issue the system also doesn't want to wake up from sleep sometimes?!.

What might be the problem with my system, is it a bad PSU or a bad Motherboard?
What best can i do to resolve this issue?

System Specs:
- OS - Windows 10 Pro
- PSU - Seasonic X-850 80+ Gold Hybrid Silent Fan Control | SS-850KM F3 (MOVED OVER FROM AN OLD SYSTEM OUT 2011)
- CASE - Be quiet! Pure Base 500 Gray
- MOTHERBOARD - Gigabyte Z390 AORUS PRO
- CPU - Intel Core i9 9900K
- MEMORY - Corsair DDR4 Vengeance LPX 2x8GB 3466Mhz 1.35V
- GRAPHICS CARD - Gigabyte GeForce RTX 2070 Super
- HARD DISK - Samsung 970 EVO 1TB M.2 SSD

Hi Evan,

Thanks for the reply.
Yes, I have a spare power supply which I can use for testing. I will check if that will solve the issue.

Still I don't understand why the system works fine for a few weeks (having no problem with a cold start) after replacing the CMOS battery?
Does this has to with the standby power failing, which causes the CMOS battery to drain faster than normal?

BR, Kaz