So it's getting kind of hot now and I thought I might just turn down the overclock.  I had some crashing in games (CODMW2R, BFV) and before digging in to that, I knew removing the overclock would be a good idea (even though temps on the CPU rarely exceeds 57c in gaming with the 2080ti around 70c).  So I did...
 
Got stuck at code A2, then after a shutdown, it wouldn't start up.  I tried resetting BIOS/CMOS with the red button.  Then I noticed the red LED by the 24 pin power connector and BIOS selector switch was blinking with a faint audible click along with the white power button blinking faintly.  It did this even with the power supply unplugged.  It stopped when I unplugged the 24pin ATX cable from the motherboard.
 
Curious, I swapped the power supply (EVGA 1200 P2) with a spare (EVGA 850 G2) and the computer booted right up without issue.
 
More curious, I put the original power supply (EVGA 1200 P2) and the computer ALSO booted without issue!
 
Anyone experience this?  Anyone think they know what might have happened?  Is a component (MB, PSU) about to fail, or was software to blame somewhere in the process of removing the overclock?  Thoughts?
 
For the record, I run the EVGA 1200 P2 in ECO mode.
 
Cheers,
Ben
 
EVGA X99 Classified
EVGA 1200 P2 PSU
Intel 5820K
Corsair Dominator Platinum 3000 MHz 16GB 4x4GB
EVGA 2080ti XC2 Ultra
Samsung 970 EVO plus 1TB NVMe on x4 PCIe card
Samsung SATA 850 pro 1TB SSD
Samasun SATA 850 EVO 1 TB SSD
Corsair H110i Pro in push-pull in front of case (x4 140mm fans)
Corsair 750D Airflow case, 1 rear exhaust and 2 top 140mm exhaust fans
ACER XB270HU monitor
Back-UPS NS 1500M2
Windows 10 1909
 

Code A2: IDE Detect..A2 is also a CMOS Checksum Error. This will occur if the CMOS has been reset or cleared. When receiving A2 code, press F1 to skip BIOS setup or F12 to enter BIOS.
 
Might be time for a new CMOS battery..Maybe there was a bad connection and swapping out the PSUs took care of it..Maybe the 1200 P2 has an intermittent problem..Hard to say.
 
I guess about all you can do now is keep an eye on it and see how it continues, or not, to work.
 
I'd also check voltages and make sure they're within spec.

If a "Cold Boot" fails ...... push the reset button .... usually the Warm boot will continue
 
depending on why the Boot got stuck ... this work very well for RAM training issues
 
other times your well served by pulling the plug from All devices & holding the start button to totally discharge the capacitors in the PC
 
also when thing go sideways .. load the default BIOS ... reboot & then edit the BIOS setting you need
 
What I like about the X99 Classy is all the BIOS profiles you can Save ... to reload later