LG C9 OLED Screen Artifact / Glitch Problem

LG 65” C9 showing screen artifacts, in a narrow vertical line (3-4 inches), on the first third of the screen, when running 120Hz content directly from my PC connected to the c9’s HDMI #1 running a RTX 3090. As an intermittent problem (happens once every few hours, sometime more) It’s been tough to capture, but I finally manged too whilst playing BioMutant. Important note: after months of troubleshooting, strangely if I physically unplug HDMI #2 on the C9 (used for ARC return to Amp only), the problem doesn’t occur?! How long have I had the issue: About 12 months. Basically ever since the LG VRR Fw drop, which allowed me to push 4k 120Hz from my PC GPU to the C9. My current set-up: 65” C9 running Fw. 05.10.25 PC GPU (Asus TUF OC 3090) to C9 HDMI #1: Maxonar HDMI 2.1 Ultra Cert cable (link) C9 HDMI #2 to Amp (Denon X3400h – Fw 7600-6145-6461-3085, used for ARC audio return only): Cable Matters HDMI 2.1 (link) Windows 10 Pro – 21H1 (all Win updates as of 3rd Nov 21) Win10 set to use RGB Full, 10-bit (have also tried 12-bit, same issue). NVCP G-Sync enabled Nvidia driver 496.49 (26th Oct 21) How does the issue present itself?: Whilst gaming at 4k 120Hz and typically (probably exclusively, though it’s hard to marry the event with the exact framerate at the time) with a frame rate above 60 and up to 120fps, I get a split second flash of vertical noise (about two frames) / artificing pop up on the screen (approx. 3-4 inches wide). It always happens in exactly the same position on the screen (left third) and the frequency varies. Sometimes I get 2 or 3 within 10 minutes, other times once or twice every few hours. Other times, it doesn’t seem to happen at all. Troubleshooting steps / supporting info gathered to date: (I’ve kept this as brief as possible, but happy to expand if anyone has any thoughts) Important Note (aka – a ray of light!): Removing the ARC audio return HDMI cable from the C9’s HDMI #2 port seems to fix the issue!?!? …but of course means no sound not only from PC, but also the C9 Apps. I did try to work around this by using Optical Audio out (C9 to Amp), but I couldn’t get it to support 5.1 for some reason.. only Stereo? I know it can’t support Atmos, but I always thought 5.1 was ok? Anyway, I digress… Amp: Assuming that cable stays in place, switching the Amp on or off doesn’t matter (aka problem is still happens). Tried removing Zone 2 HDMI output cable (feeds my Phillips Hue Sync Box) Tried changing the Amp from ARC to EARC, but only shows ARC as an option Tried changing audio outputs (Stereo, 5.1, etc) Tied updating Fw (no new firmware available) General: Happens in HDR and SDR Happens when gaming above 60FPS Frequency of occurrence is random (could be a few times in an hour, once every few hours, or not at all for a day or so). C9: Have changed many (perhaps all) of the C9‘s settings in isolation (see nuke option below) Tried changing the C9 Audio out settings (HDMI ARC) to Passthrough, PCM, Auto. Also tried EARC on and off. Disabled LivePlus Tried PC Mode (PC icon and tag) Tried Console Mode (aka, not PC mode – lol) Disabled OCF: press and hold ‘input’ button to call up input menu. Click ‘cog’ icon Ultimately – factory re-set the C9 (took the nuke option) GPU / NVCP: Have DDU’d the Nvidia drivers (many times) and of course updated them over the past 12 months Upgrade to V3 and them V4 Bios (basically, just added ReBAR support) Removed Overclock Tried both HDMI outs on GPU Can’t try Display Port (hopefully we’ll get one on the C2 ) Tried forcing V-Sync in NVCP Tried capping fps to 120 and 117 in NVCP Disabled G-Sync support for Windowed mode (i.e. enabled ‘full screen’ only) Cables: Have replaced both PC to C9 and C9 to Amp HDMI cables three times (twice with fully certified Ultra HDMI 2.1 cables, when I stopped being a tight arse). Software: Disabled Afterburner Disabled RivaTuner Tried an Rivatuner cap of 120fps Tried an Rivatuner cap of 117fps Disabled all game overlays (i.e. Nvidia, Uplay, GameBar, etc) Windows: Updated from 20H2 to 21H1 (and have taken every incremental update) Change desktop refresh in windows (not NVCP) to 119.8Hz Output to headphones, leaving the LG C9 Audio HDMI output enabled (but inactive) Output to headphones, disabling the LG C9 HDMI Audio output The unplugging of the HDMI #2 input ‘fix’ I stumbled across by pure chance a month or so back, really had me stumped… it also makes me think there may be some kind of interference coming back up from the Amp. Almost as if it doesn’t like the 120Hz feed?!?! That makes perfect sense of course, as the Amp only supports HDMI 2.0 (4k/60)… but surely an ARC return from the C9 to the Amp is only pushing out Audio, not Video (which is precisely the reason I had to go PC to C9 + C9 to Amp (ARC only), instead of PC to Amp to C9.