Prepare to go down the rabbit hole. I have tried every trick I have found on the forums.
Running games in full screen and borderless, with Gsync compatible mode on and off.
Disabling Windows features like game bar, game DVR and HAGS.
Caps at 60 or 120 fps, with and without changing the monitor refresh rate.
Enabling "Limit capture framerate" in Game Capture settings.
Lowering game settings to achieve smoother framerates and rule out encoder overload.
Various tweaks to NVENC settings.
At some point I thought maybe it's just my monitor so I tried watching one of my streams as a VOD from Twitch using Apple TV connected to my 60 Hz 4K television. Didn't look smooth at all.
The VODs look okay(ish) only when viewed on a phone.
No frame drops shown in OBS stats while encoding, whether it's streaming to Twitch or recording to an internal SSD.
No notifications about anything in the logs when sent to the analyzer, except when HAGS is on, but I've left it on since disabling it didn't change anything, I have an RTX 4070 Ti and DLSS frame generation does not work without HAGS.
This seems to be a common issue and despite everything I've read I'm still not sure whether it is just bad luck - a very specific problem with some builds - or a problem in Windows and/or OBS.
Good luck, friend.