55 Inch LG C7 OLED Erratic Behaviour with HTPC

[NeonBlizzard]

Finally got around to using my old desktop hardware to put together a nice little HTPC for my home theater. 

Specs of the PC:
Intel Core i7 7700k @ stock clocks (have a low profile CPU cooler so don't want to push it like I did when it was in my desktop rig, didn't realise just how big the GD10 was, could have got something beefier)

ASUS Strix Z270-E Motherboard

ASUS Strix 980 4GB

32GB HyperX Fury 2400Mhz RAM

Corsair AX860 PSU

All wrapped up in a Silverstone Grandia GD10. 

 

And the TV in question is a LG C7 55" OLED. 

 

Running certain resolutions causes graphical issues such as little green dots (I guess small groups or even individual pixels). Kind of sprinkling over the screen. Not very dense but noticeable - especially against the solid black background. Also when this is happening, the display will often drop out, then return. And eventually drop out again. 

 

I thought perhaps I was using an older HDMI spec cable (was one I had laying around) and used one from my Xbox One X. (Note that I have a PS4 Pro and Xbox One X and both work flawlessly. 4k HDR and I have used my current desktop w/ 2080ti before just to see how well I could run 4k and there were no issues).

 

Even with the for sure higher bandwidth cable, the same issues happen. 

 

The ONLY resolutions that appear to work correctly without any issues are: 

1080p120

2160p30 

 

All other resolutions I have tried, including lower resolutions give the above issue. 

For example, 1080p60, 1440p60 and 2160p60 have the issue. Even 800x600! Occasionally when switching to one of these resolutions, the display just drops out entirely, only to return when it restores itself to one of the above functional resolutions.

 

Note that ALL of these resolutions function the same whether HDR is on or not. Prefer to have HDR on however, kind of the point of the OLED.  

 

Now it doesn't make sense to me why 1080p120 works fine but 1080p60 does not - I mean, I don't want 60Hz over 120Hz but the not working just doesn't make a whole lot of sense. 

 

Has anyone else experienced anything like this? 

Any troubleshooting tips for either the PC or the TV? 

[circeseye]

ok you have to remember this is a tv, not a monitor. your trying to make the tv use resolutions it does not support

1080p @ 60Hz @ 4:4:4 Yes

1080p @ 120Hz  Yes (native support)

1440p @ 60Hz  No

4k @ 30Hz @ 4:4:4 Yes

4k @ 60Hz Yes

4k @ 60Hz @ 4:4:4 Yes

4k @ 120Hz No

 

All the common input resolutions are supported. For the TV to properly display 4:4:4 color, the HDMI input in use must have its icon changed to 'PC'. To play 4k @ 60 Hz @ 4:2:2 or 4:4:4 color, 'HDMI ULTRA HD Deep Color' in the general settings must be enabled for the input being used.

Update 05/31/2017: 1080p @ 120 Hz + HDR does not work properly. Its brightness appears to be following the SDR gamma curve rather than the HDR PQ curve, making everything look off and too bright.

 

https://www.rtings.com/tv/reviews/lg/c7-oled

 

also need to make sure your plugged into the right hdmi on the tv on some units.

and have seen tv's be picky about hdmi cables used.