Same here, RtX2070 Win10, LGCX. VRR works randomly only for seconds/few minutes, then it stucks at 120hz, permanently stuttering. Impossible to enjoy the game that way.
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After literal hours of pulling my hair out trying to work out why VRR would sometimes work and sometimes not (using Razer blade 15 advanced connected via HDMI 2.1 to an LG CX), I finally found the answer (at least for me). It’s the NVIDIA FPS overlay- when it’s on, it seems to interfere with the game’s ability to transmit a VRR signal. Just turning it off (by pressing Alt+R) solved my problems and switching it back and forth reproduced the issue finally confirming to me that this was the cause of the problem. Hope this helps others from losing any hair over this!