meep

So I have an update;

Came to my desk this morning and my server was still chugging away, not asleep. I checked the system log and it's clean as a whistle.

While I don't have a definitive action I took to effect this change, I did encounter a significant problem since I last posted that's more than likely related if not the root cause: a failing boot thumb drive.

Through the various restarts I conducted yesterday to check BIOS settings etc., I started experiencing GUI lockups of increasing rapidity a couple of minutes after restart. Checking the logs, I noticed reports of USB access issues leading to errors with device SDA, my boot drive.

I tried swapping it around between ports, changing between efi and non efi boot etc., to no avail. I bit the bullet and transferred to a new boot drive (my 4th in 16 years, not bad going). Boot up was noticeably snappier and the USB errors stopped immediately.

It looks like my USB boot drive had been on its way out. I was possibly missing the errors as they were getting lost in log spamming from another failing unassigned device in my older system, and I'd implemented a user script to clear syslogs overnight to prevent the system crashing from full logs.

I'm not sure how the failing USB caused the system to go to sleep overnight, and maybe its unrelated. I'll keep everything as is for a few days for observation before I attempt an upgrade to 7.1.x. As this system has had a complete heart & lung transplant (MB, CPU, RAM, GPU, Case, Cooling, Some drive swaps) I would expect little niggles here and there so will remain on high alert.

Fingers crossed.

Thanks for all the input.

So we might have a winner here.....
 
It's an oldie, but a goldie. Working through any and all possible troubleshooting, I disabled c-states in my bios and have had my first error free night in several weeks 
 
Not fully confirmed yet, and super confusing why this would impact my system now and not several years ago when this was actually a thing.
 
Will report back and hopefully close out the thread if this turns out to be the actual solution.