[Bibio]

now got it playing for days or no dropped/repeated frames with 4k hdr, bluray and dvd :-)

Warner306 is correct in saying, dont touch the reset gpu button while configuring custom modes.

i also think that changing modes while in the nvidia cp helped as well, also changing to rgb, 12bit and full helped before configuring the custom mode.

also got rid of the sunburnt skin tones by changing the LG tv colour gamut to auto instead of wide, extended.

I take it you disabled color calibration controls in madVR? In that case, madVR would output BT.709. You could try setting the calibration to BT.2020 or DCI-P3 and setting the LG back to wide or extended, but having it convert to BT.709 when SDR content is encountered would be more accurate than upconverting from madVR to DCI-P3. The calibration setting in madVR only effects SDR mode, not HDR passthrough, FYI.  

 its actually HDR content that is the problem. i have noticed that its dependent on the nits value of the video when it hits 4000+nits it turns skin tones to something resembling sunburn.

i thought i had found a solution by using filename rules e.g. video name profile=UHD1.mkv but i soon found out that you cant use profile auto select rules at the same time so abandoned it. the profile=xxx worked but it meant that i would have to rename every single file in my collection which would be a major task. if i do decide to do it then i could control the PQ of every single video i have in my collection by creating group profile/profiles but its too much work.

if there were a way to use profile auto rules boolean and filename rules at the same time then i would be in heaven as i could set a profile for each nits value of hdr files not to mention more control over older SD content :-)

if there was a way to write boolean along the lines of, if (srcWidth = 3420) and (HDR) and (nits >=4000) "HDR1" then that would be even better but i dont know if madVR recognises nits values in its boolean logic.

i'm slowly beginning to learn that madVR is a very very powerful rendering tool that has a lot more functions that meets the eye.