Okay, it seems that whatever the settings, MC is sensitive to other windows scrolling text and selecting items. (Or scrolling text and selecting items in MC itself.) Seems typing will do the same thing. More sensitive when 5.1 is enabled, 'it SEEMS,' but it still does it at default settings.
Also, when 5.1 is enabled it's much more likely to 'hiccough' when it starts buffering the next song in a playlist.
I figured out what was making Playing Now flicker... I had my sound up, and the vibration was pinging the mouse... MC saw the mouse movement and unhid the scrollbars.
So, that's what I've found. I pegged the processor to 90% by using the fireball visualization in windowed mode at highest display setting... (with 2 speaker stereo... can't test that in 5.1) sounded fine even between songs as long as I wasn't doing a lot of scrolling, clicking, or typing.
Edit: When in 5.1 mode there is ALMOST ALWAYS a small skip between songs.