I´m sure you know this but I add these settings tweaks for you to try.
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Change the Vertical sync to "ON" in the Manage 3D settings in NVIDIA control panel.
Also set (in madVR, under rendering - Windowed mode) present frames in advance to 1.You can also try to disable (in madVR, rendering - general settings) use Direct3D 11 for presentation.
Try experimenting with queue sizes as well, I have had good results when increasing the CPU queue size to 24 and GPU queue size to 16.
The first two solved the micro stutter for me using a 2080 graphics card.
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To fix stuttering:
* madvr settings > general settings > windowed mode / exclusive mode > frames presented in advance = 1
* NVIDIA settings > Power Management Mode = Adaptive / Normal
* NVIDIA settings > Vertical Sync = Adaptive
* NVIDIA settings > GSync = Disabled globally (use profiles for games)
* NVIDIA settings > disable Multiplane Overlay
Multiplane Overlay:
https://nvidia.custhelp.com/app/answers/detail/a_id/5157