I have a strange issue where my PC works great as long as I don't shut it down.
I initially suspected faulty PSU and replaced it - no joy.
I replaced the cmos battery - no joy.
I even bought a new case because I suspected the problem might be with the front panel connectors - didn't solve the problem but the new case is nice!
Basically when it happens - I have to turn off power, unplug PSU, press CMOS button.
After this ritual - the PC will sometimes power on momentarily and then immediately shut off. The BIOS light on the mother board flashes on and off.
Then I'll get lucky and it will all power up and take me to the bios/uefi to reset and reboot.
When I do get it booting, as long as I don't power it down, the PC remains rock solid. I have performed all sorts of stress tests and played demanding games for hours without issue.
I'm at a loss as to what's happening.
On inspection, I can't see any bulging caps or anything on the mobo.
Next time you shut down and it won't boot - disconnect power lead to PSU or unplug from mains socket. - Press and hold power button for 30 seconds - reconnect to mains lead and see it it will boot normally.