Hm...sad to read all of this just to realize that I'm encountering the same issue.
Everything was working perfectly when I last streamed in September 2020, took a longer break and wanted to stream just for fun a little here and there.
All of a sudden my Ryzen 9 3900X can't handle x264 encoding any more, even veryfast has some regular stutters (slow preset was perfectly doable in Sept 2020...)
Can upload some logfiles later when I'm done with work and I can tinker around a little bit more.
Only thing that changed was the Windows Update (2H20) at first, then I started to update my nvidia driver and OBS version to see if it changes anything.
I'm sending my video signal via NDI (OBS Plugin) with my native resolution (2560x1440) to my stream rig.
Canvas and Base Resolution there is 1920x1080 and then I rescale my Output via the encoder to 1536x864 @ 6000kbps.
This has worked like a charm before. I don't have any issues in my preview on the stream rig (game rig disabled), except when I start streaming/recording with x264.
Was also able to reproduce the problem constantly by using the same settings for recording as I stream with. (x264, CBR @ 6000kbps, keyframe 2, CPU usage slow/medium, Profile high, tune none and no additional settings).
Stutters all over the place, lowering the CPU usage preset to veryfast does help a little, but the issue is still there and noticable.
I need to verify this, but I think I don't have any issues when using NVENC..funny side note:
If I stream and save a replay of 15 seconds length with NVENC, I don't have any stutters in the video, but I do have it on stream.
Maybe something is amiss with x264 and the newest Windows Update?