Thank you for the thoughtful reply.
I realize now that I left out a very important point.
None of the avi or mkv files that cause the crash on playback are in 3D format.
They are simple good old fashion 2D content.
ahhh ... got it
i recall this problem ... if 3D setting is left on then 2D video crashed MC (in those days Windows driver had bug that prevented it to toggle 3D setting of Nvidia) ... not sure what caused crashed MC or MadVR ... that bug was fixed (Windows driver abke to toggle Nvidia 3D)
why you manyally switch on 3D setting in Nvidia ... 3D setting in Nvidia and Windows should be off ... let madVR turn it on and off when needed
in MC ... options -> video -> Open MadVR Settings (remember that MC uses its own copy of MadVR ... so must access it through MC)
in MadVR ... rendering -> stereo 3D
enable stereo 3D playback (tick)
when plaing 2d content (tick) ... disable os stereo 3d support (tick)
when playing 3d content (tick) ... enable (tick)
restore os stereo 3d settings (tick)
These settings in MadVR guarantee that 2D video is never playbacked with 3D setting on ... and restores your 3D setting after video finish playing (in case you use it for gaming)