Stuttering starts

[MJDNordEst]

Hi - I have a problem that is completely baffling me.

Build:

• AMD Ryzen 5800x CPU (Watercooled - custom loop)
• Gigabyte x570 ZORUS MASTER
• AMD 6900 XT GPU (Watercooled - pre-cooled card fitted into custom loop)
• 2x16GB RAM
• 2x Samsung EVO SSDs and 1 SATA drive for storage
• 1000W Plat vert PSU (Now 18 months old)

My problem is that when I press the power button one of several things can happen:

1. PC boots up absolutely fine - benchmarks solid and have remained in the same range without appreciable deterioration for months
2. Fans spin, but little else happens. A single tap of the power button and system shuts down again.
3. PC boots, but very slowly. as windows opens the dots circling move slowly and freeze from time to time. takes 10-20 seconds for windows to start. When it does mouse sampling and keyboard response is very poor and is essentially unusable. Have to restart
4. PC seems to boot up fine, but the display believes there is no signal and therefore turns off. Have to hold power for 6 seconds to force shutdown.

Sometimes it boots up fine several fays in a row, other times I have to try 10 or more times before the system boots and opens windows as it should. I have already tried the following

• Clean reinstall of windows
• replacing hard drives
• trying a different graphics card (an air-cooled Nvidia 2060 - same problem)
• replacing the CMOS battery
• having all power setting in BIOS lowered, disabling XMP etc.
• taking it all apart and putting it all together again
• removing panels from my case to ensure not a cooling issue - though it often fails to boot from a cold start in any event ?!?

None of these things have made any difference - I am now considering buying a new MOBO but thought I'd see if anyone else has any other ideas first?
When the system does start ok, then it works as it should throughout - it seems it either goes wrong during start-up or there is no problem at all. If it starts properly I can work the system hard for hours at a time without issue.

Any suggestions on how to rectify this short of replacing every component bit-by-bit?