This is always kinda confusing to me because there are so many variables involved. I have a similar build (same monitor! So at least you know it should work), my settings, which seem to work correctly, are GSync on in the NVIDIA Control Panel. It sounds like you did this, but just to check, there are two places, one is the Set Up G-SYNC section, where you make sure it's enabled (oh, and make sure it's enabled for whatever mode you're using, if it's enabled for full screen and you're using windowed, that would be a problem). The other is in Manage 3D settings where I have it set on global settings. Maybe also check to make sure it didn't somehow get disabled for specific games in the Program Settings. Like you, I have vertical sync on in control panel, but also make sure you turn it off from inside the game, in its options. The game's VSync may not play nice with NVIDIA's VSync, and you don't necessarily know how any given game has implemented it. Triple Buffering is off in the control panel. I have power management on optimal power, I'm not sure if that's the default if I put it on there on purpose, maybe one thing you could try is max performance, the issue could be your gpu trying to aggressively downclock but then suddenly need to speed up again and that makes it hitch. I'd set the program settings to max performance because if you do it globally I think you'll run real hot just browsing, but I'm not sure.
The fact that it's smooth at 60 fps seems to point to the issue being G-Sync not working, since your machine probably just pegs the game at a steady 60 fps. The whole point of G-Sync is that we can't play at a steady 144 but it keeps things smooth, so hopefully you figure out the problem!
Edit: What were you using to limit the frames? I've never messed with that but in theory using like MSI Afterburner to set your max framerate at 60 should have worked (although that's not a desirable solution, it should still be the same effect as switching to the HDMI port).