temp related could make sense, if the RAM literally freezes/pauses for a moment, then wakes up literally a second later - one brief bit of stutter and its cool
whys it erratic? cause that one ram module simply may not be the one in use, the next time
The only thing I can think of is that it is some sort of bizarre never-before-seen "cold bug" on Hynix. If it chose to manifest, it would only be in the first half of the first match of the day. If I saw it and immediately quit and restarted, it would continue as soon as I came back, and would persist as long as I didn't suffer through the always initial 5-10 minutes of gameplay before it went away.
As you know RAM always starts out cold in the high 20s or low 30s, then slowly makes its own heat/roasted by the CPU/roasted by the GPU. MW is not a light game, always hits both CPU and GPU hard (even if framerate limited/DLSS/CPU or GPU bound). That said, either no other game suffered the same way or was never obvious enough to notice.
But MW isn't that much of a RAM heater, the CJR @ 1.38V was like 44C max. Almost the same temp as the B-die @ 1.5V, 43-44C. It's not nearly enough to trip B-die, which is by FAR the most temp sensitive of any IC.
For a long time esp. on the cursed 3700X I thought it was an IF issue. But it seems like a red herring in the end. I did have the random USB disconnect sounds from Aug 2019 all the way up till I switched to the Unify-X last month, but that's separate.
Hm, that is really interesting. I've been seeing some ... possibly similar weirdness on my system, in particular packet loss and latency variation warnings in Rocket League (it's the only online game I play with some regularity). Seems to happen regardless of the internet connection quality. This is with my 3200c14 B-die though (running at DOCP, still haven't bothered to OC it any). I might stick my old TridentZ kit in here just to see if anything changes.
As part of troubleshooting this, I've discovered that my system seems to get some pretty bad DPC latency issues after one or more sleep cycles, particularly when left asleep overnight. After a reboot everything is fine though. Any chance you could run LatencyMon with your previous kit after a sleep cycle or two to see if there is any relation? (I've been in contact with ASRock about this, and they say they can't reproduce it.) My RAM never exceeds ~45°C though.
I tried running LatencyMon a lot. It would always pick up on the nuanced micro-stuttering due to multitasking/monitoring software/other GPU usage while in game, but never on the extremely obvious frameskipping. Not once. LatencyMon wouldn't even register a small spike, just looked completely normal though I was literally missing the first 0.1 sec of a slide or inspect animation.
On the topic of the other microstuttering, moving to dual rank B-die was a CRAZY difference. The perceived reduction in input lag was like 60Hz>165Hz all over again. And not even the slightest hint of stuttering, I never realized how rough it used to be until I fired up the game on 3600CL14 and it was BUTTERY smooth. I could entertain the idea of placebo if it was at night or drinking (tired, less alert, everything feels "smooth"), but this is in the middle of the day.
Both are dual rank kits, so it's not the single rank min FPS issue. 3600CL16 CJR to 3600CL14/3800CL14 B-die.
Although, I'm debating going back to 3600CL14 because 3800CL14 doesn't actually improve anything in any game, lol. Saves me 80mV in VDIMM, 75mV in VSOC, 50mV VDDP, and a whopping 110mV on VDDG