Wow, thanks for all the feedback and suggestions. Lots to think about.
@JimH:
Do you mean Red October HQ?
Yes, I mean HQ. I am running the latest version on all three machines, only having trouble on the mac mini and only with the standard renderer.
@CountryBumkin:
Did you know that you can setup the Video Mode>Advanced>Custom so that the mp4 file can play with a different renderer. Perhaps use MadVR instead of EVR for only Mp4 when in RO-Standard. I would also suggest that you reinstall MC just in case soemthing installed wrong (as you mentioned in your post) - since this is easy to do.
I did not, and that's very cool to know - that's a decent short-term solution.
Did you try installing the latest nVidia drivers?
I'm not sure where to get them for the mac mini - seen reports that the standard nvidia ones don't work, and before I make the wife angry by taking out her media machine, I wanted to try low-hanging fruit first
What are your machine (mac-mini) specs and which nVidia gpu does it use?
It's the 9400 on-board card. This is the mac mini that doesn't have the HDMI out.
Do you have hardware acceleration on?
If you mean within MC, no I do not. Turning it on makes the video even worse - it's all garbled and "swirly" almost like when the digital cable box screws up
Reinstalling MC hasn't helped.