Sporadic system hangs/freezes, no BSODs or recent major HW/SW changes

A bit rambly, I apologize, but there's a TLDR blurb at the end.

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A bit under two weeks I got on my computer in the morning and it was acting semi-frozen. I was able to swap between one or two windows before everything stopped responding. Throughout this I had full control of the mouse, and it changed shapes based on what it was mousing over pretty much the entire time (don't remember for sure so I won't swear to that). The system clock stopped updating as well, during this. After a few minutes, stuff started responding again. I rebooted the machine once I was able to and update a program that had been nagging me for a while; during the update process there's another few minute hiccup, and when that ends everything goes back to normal for the rest of the day. Run whole pile of system scans (MalwareBytes, Spybot, and both a quick and full scan with MSE) and they all come up clean, as well as checking on Windows updates (had issues a while back with wuauserv eating 10-20% of my CPU so I disabled the automatic checking).

Next morning, same symptoms. I run sfc /scannow, it comes up clean. chkdsk /r comes up no issues as well on my OS drive.

While nosing around looking for solutions I find someone with similar symptoms finding the source of their problem being a particular file was giving defrag programs fits and deleting it fixed things, so I start defragging on the off-chance that finds anything. I do this periodically anyhow, but it seemed worth a try. Defragging D: and F: proceeds over the next few nights (one backup program makes ginormous files that take forever to consolidate and another one likes five million smaller ones) and on every night the defrag is running overnight the computer is fine in the morning.

Then I forget to set a defrag running Thursday night and it's back to stalling out Friday morning. Reboot... and now it starts lagging right off the bat - not sure if this is new or just a percentage chance on a fresh boot. Log onto a second account, which doesn't lag, and write down what all's listed as running in the Task Manager and what non-MS services are showing as well in the system config window and compare them to what's showing in my usual account - the file's still on my desktop which is churning through Memtest86+ so I can't list precisely what the differences were, but the services were identical and the handful of differing processes mostly looked to be system stuff. Upon logging back in mid-afternoon everything ran fine the rest of the day until I closed World of Warcraft for the night at which point it started lagging out again. Before heading to bed that night I log into the second account (also admin account, virtually unused and untouched otherwise) and leave that up overnight. Second account is stalled out the next morning so at least it's (probably) not something tied to my account specifically.

Machine spent most of Saturday running Memtest86 (not +), it completed a full 4/4 pass with no issues. Found a support thread elsewhere talking about a possible fix people had found for safe mode getting stuck in a reboot loop, which is something I've had issues with for a while (when *that* started I don't know, I normally never need to use that) but it doesn't work. Manage to get into safe mode anyhow, by dint of crashing the computer while it's trying to boot into safe mode and getting the extra options version of the boot menu and selecting "Last known good config" option (this seems to be a reliable method, even if it is cludgy and broken as heck). Leave it up in safe mode overnight and there didn't seem to be any issues this morning. Don't know if this was the same flavor of safe mode the system normally tries to boot into though, a lot of the MS services were disabled and don't know to what degree that's typical.

Main reason I'm running Memtest86+ now is because it's still stubbornly trying to get into safe mode and I decided to be lazy and boot off the CD I'd put in earlier rather than break the cycle. I figure once I do boot back up normally it'll be time to play Christmas lights with start-up stuff, but that's a rather daunting task given that I can only reliably reproduce the issue by leaving my machine idle for several hours (brief periods haven't caused issues so far, including up to ~3 hours on one occasion). (Also possibly todo, though I'm a bit wary of this sort of mucking around: update BIOS. Never needed to do that before but there've been two updates since I bought the board and don't know what version it was packaged with. Had no issues of this sort for a few months of normal operation though, so...)

Additional points of note... While trying to pinpoint issues with my graphics driver a few months back I'd stress tested both CPU and GPU and they came up clean (not that I'd expect the latter to be causing lag, but...). Running system monitoring software during that same period had nothing that stood out as unusually high temperatures or peculiar numbers (HWMonitor was giving an erroneously low voltage for I believe it was the 12V on the motherboard, like 7 or something, which seemed too low to be real given that the PC was functional, but HWiNFo64 or however it's capitalized showed a believable value a bit over 12V). Installed CrystalDiskInfo a few days after the first lagging incident and the only time it pinged about anything was an external getting warm and that was more a cautionary ping than an actual issue - every drive is showing as good condition. Sleep/hibernate power options were all disabled ages ago (suspect sleep issues scrambled the brains of my previous motherboard) and pretty much every other power saving option (USB, monitors, idling the HDs, etc.) had been disabled at some point prior to at least one of the lagging incidents. No hardware changes in the past few months and the only software changes have been a couple of updates, one of which (IDrive) I uninstalled midway through the mess for unrelated reasons (will be reinstalling it later).

On none of these stalling outs were any error reports/events generated that I could find, save for one time when I managed to get Explorer.exe to realize it wasn't responding and offer to restart - can post the event description text I copied, but I did a bit of searching and it seemed to be a generic error message.

Can update system specs (working off memory and stored receipts, since PC is busy with Memtest86+ atm) or post some flavor of spec program info later but CPU/motherboard/RAM are all ~5 months old and the SSD is somewhere in the same vicinity (forget precisely when, but predates Win10's free upgrade period ending by at least a little while), but everything else dates back to the original system build a bit over four years ago. Windows is more or less up-to-date (think I installed everything non-optional on the check last week, but GPU is on an older hopefully-more-stable build (362.00) since more recent ones were sporadically rebooting the driver on me.

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TLDR - Sporadic lagging out of the system, but never losing control of the mouse (which differs from a lot of similar reports I've seen online) and no errors/events or BSODs resulting. Usually after sitting unused overnight, but not always. All individual bits of hardware seem to be checking out clean and system scans report no issues. Given all that, and the issues with safe mode, I'm probably going to run a second* in-place reinstall and hope that this one sticks, but figured it was worth poking here to see if there were any other suggestions of things to look into before I make the jump and potentially wallpaper over the actual cause because I've kinda' run out of ideas.


(*wuauserv was going haywire and Aero was crashing after replacing the motherboard/CPU/RAM back during the summer, so I ended up in-place reinstalling which sorted that out and have had no repeats of either since.)