This just happened while I was downloading updates.
I believe, this time, Discord was somehow not completely “loaded”. The un-dip came after I shut Discord down, and the entire PC started going on smoothly once more.
This just happened while I was downloading updates.
There is a program in Fedora Linux called systemd-oomd.service. It will start killing things if the system starts having memory problems. 
Have you tried monitoring your memory usage when one of these stuttering and/or reboot events happen?
The problem is that when “it happens” the computer becomes either functionally or actually unresponsive, and if it doesn’t restart, then later on nothing can be seen (unless you’re a wizard and know everything ever).
I’ll try out that systemd-oomd.service thing tho.
Still:
Maybe it’s something wrong with the hardware, but other than the fact that I don’t believe it’s a BIOS problem, so I won’t risk bricking my MOBO to update it,
this doesn’t happen during anything critical,-
-it usually happens just after booting, while getting Brave, Discord and Steam up and running, and when updating Steam games (which are on the boot SSD).
This time Discord was struggling to “load/function”, so killing it “fixed” the problem (when Steam is finishing or changing a game’s download, the small choke in-between stutters the PC, with even YouTube videos freezing on one frame, while the audio still plays, and the video technically still playing).
I’m not sure what Btrfs does, but XFS mentions differences in how metadata is accessed with multi-threading and other multi-threading advantages, which implies it might be affected by CPU load and scheduling more-so than a filesystem not doing that (ext4).
Hence ext4 on / and/or wherever Steam games are stored might rule out something Btrfs might be doing.
I haven’t used Btrfs, but haven’t seen large file I/O and Steam validating/downloading causing system freezes or hitching with ext4 or XFS.
I’d try ext4 first though (there was reported problems with XFS and Steam games like L4D2 and something like 64-bit inodes)
It happens even on Windows when just using Brave Browser (I believe I said it somewhere above here).
I do not know what the problem is, but it seems to be limited to this PC alone…
Did you try disabling IOMMU and CPU virtualization? If dmesg reports anything with iommu, IOMMU, or DMAR it’s probably on and remapping devices (which could be slower/affected more by load with the IOMMU having to now process high I/O from drives + etc likely GPU/DE rendering)
Edit: Windows will do the same kind of remap if Virt/IOMMU is BIOS-enabled (but quieter unless you’re checking paths under GUI Device Manager)
Could always want what is reading and writing to disk by running iotop.
You earlier said “sometimes the machine shuts down and restarts” and that should not happen in normal operation. Running out of ram, and then filing swap would look like the situation you describe though.
I appreciate the advice, but, other than the fact that I am ill, so I am not gonna touch my machine while my brain is foggy,
this is an issue which happens under W10 too (in fact, under W10 it’s more common for it to just reboot, without giving out any error ever),
so I do understand if these are “diagnostic commands to see what the hardware is doing”,
but (with all respect) “any solution” which ignores W10 will probably be wrong.
If theres any similar issue under windows 10, it will be logged in the Event Manager.
I’m also not offering a solution - I’m telling you about a tool which will show you what (and how much) is being written and read from the disks. You can choose to look at it or not, but if you want to try to determine the root cause, gathering evidence is step 1.
Have another look when you feel better. In the meantime it probably won’t get any worse, unless it’s a hardware issue in which case you’re somewhat buggered if you don’t have backups of anything important on that machine.