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New stability puzzle

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Mar 7, 2008
System key parts:

i7 6700k relatively early CPU bought soon after launch, not delidded, not run for any significant amount of time above 4.2 GHz
Asus Z170 Maximus VIII Hero - bios 3802, latest
Ripjaws V 2x8GB 3200C16 early version that is dual rank
Corsair HXi 650W PSU

This was my main system from around 2015 until replaced by Coffee Lake. I keep it around as there's a bunch of software and stuff on it I'm too lazy to transfer to newer systems. It is still used daily. Yesterday, when I wasn't using it (idle on desktop), it suddenly powered off. Odd. Didn't think more of it. Today, while watching a YouTube video, it crashed. Hard lock, desktop still visible, glitched repeating sound playing until I reset. There is probably a deeper problem. I went into the bios. While poking around the settings, it locked up again. Ok... another reboot, I turned off XMP as the only thing I can think of doing quickly, save and exit. So far so good...

Don't mind about performance for now. It doesn't do any heavy lifting. If XMP off makes it stable again, I'll leave it. But the system has been running fine for years. Is a component ageing and pushing it over stability edge? Which one?

Some quick verification/diagnosis, with XMP off:
Disk SMART good
Windows integrity self check clean
Prime95 small FFT - temps below 70C so within expectations. Interesting, with XMP off it seems to be using 95W long term power limit. Think it was unlimited with XMP on, MCE off. Peak temp around 70C at 110W, low 60's after dropping to 95W.
Running Aida64 mem test now, ok for 5 minutes
Prime95 blend - ok for 10 minutes

Kinda wonder if it is CPU, ram or mobo. Maybe even PSU?
Only way to narrow it down is to test with other parts. It does appear to be memory, possibly IMC. I'd toss another RAM kit in and see what happens.