Consolidating some commentary to this thread.
For about the last month my Z690-G system has become unreliable; the typical failure mode is that I get up in the morning and find that at some point in the night, the machine has stopped responding, and the typical time of day that it has stopped responding will be 4-5 AM. When this happens, all USB devices with LED lights are turned off, and the only thing lit anywhere is the power LED.
I *am* able to hold the power button down for 5 seconds to get the system powered off, and then if I push it again, it will immediately power back on.
This latest time I decided to pop the side of the case off and take a look: all the fans had also stopped, all three chassis fans and both CPU fans.
I've done all the usual song and dance of unplugging all the external hardware except the keyboard & mouse, going back to a very basic Windows installation, ruling out every software possibility I can think of, etc. I've run a RAM tester for 16 hours with no issues, beaten the heck out of the CPU with Cinebench and Prime95, and these tests have found no problems.
I kind of suspect now that it has to be either the power supply (a 1000W Seasonic Prime PX-1000) or the motherboard. I can't imagine why it takes 3 nights for the trouble to reveal itself, but that's the behavior.
Any other ideas?