Unfortunately, the system was running fine for 2 years and then deteriorated to not-quite-fully-dead in the span of about 20 minutes.
It would only attempt to POST 1 time out of about 10 power on attempts and won't finish enough of the POST to even make it into the BIOS/UEFI setup. When it would fail to even attempt to POST it wouldn't even initialize the USB peripherals (keyboard doesn't light up).
I swapped everything (GPU, ram, power supply, etc) before getting to the point of suspecting the motherboard was faulty because it would not Q-FLASH via a USB thumb drive so I sent it in for warranty service. Since the manufacturer said the motherboard passed their quality checks there is nothing else that could be left besides the CPU.
Even given all that, I refused to believe the CPU could be the culprit until I saw the failure rate graph for AMD 5000 series processors in the article that far exceed what I figure would have been in the 0.5% range.
Live and learn I guess but it certainly adds a new fun dimension to troubleshooting because I don't exactly keep spare CPUs laying around "just in case" like I do with a spare power supply.
oof. Still, hopefully other people read this and try disabling C States. I was about to spend the $25 for a spare CPU when the C state issue came up and...
For me it was simple:
disable C states - Stable for a week (at first it'd be weeks between crashes, then days, then daily, and at this point a couple hours)
Enabled them again - crashed within an hour
Disable again - Stable for a week.
Flop back and fourth a handful of times in the same day.... same as above.
I can only guess that this might be something that was fixed in the mysterious "frequency voltage curve change" stepping.
