I've been plagued with this micro stuttering issue myself for quite some time and have tried a lot of things but nothing worked (except one).
Basically, I had the problem for a long time, but I thought the problem was with my older GPU (had a GTX 680 - 2GB) and my "low" system memory (had 8GB).
But the random stuttering problem persisted even after upgrading the memory to 16GB and the GPU to a GTX 1660 TI.
Dropping most of the game's settings down to zero or disabled/low was the only way I initially found to bypass the stuttering problem.
However I noticed that, when running PZ, some of my 8 logical processors where peaking to 90-100%, while others where mostly idle.
Long story short, I disabled "hyper-threading" in my BIOS cpu settings, and the CPU usage became much more consistent (all 4 processors/cores where now similarly used around 60-70%) and the stuttering all but vanished. I only get a bit of lag when the game starts, maybe 30 seconds in, then everything runs (mostly) smoothly, with very very rare stuttering (usually when loading new map chunks).
These are my current game settings:
Fullscreen:Yes / Borderless Window: No / VSync: No / Multi-core: Yes / Post-processing: Yes / Resolution : 2560x1080 / 3D Models : All / 3D corpses : Yes / Blood Decals :100% / Lock Framerate : Uncapped / UI Offscreen Rendering : Yes / UI Rendering FPS : 15 / Texture Compression : No / Double sized textures : Yes / Lighting quality : High / Lighting updates : 15 / New roof-Hiding : Yes / Enable Zoom :Yes (200% , 75%) / Wind sprite effects : Enabled / Door shaking on hit : Enabled / Dynamic skybox : Yes / Environment reflections : Yes
Just for reference my pc specs are:
i7-3770K @ 3.40GHz,
DDR3 16GB @ 1600MHz,
GTX 1660 TI
PZ installed on Kingston SSD
Win 10 x64 (1809).
PZ 40.43 (GOG), single player
Hope this helps.