Just wanted to chime in that I have this problem as well. While it may be true that this only occurs when connected to the internet, my cable modem is "always on" so this doesn't help me much.
I would also like to point out that the studdering effects most of the application. For example, mouse over/hover effects, menus, and scrolling will all pause when the visualization does and is very anoying (music plays fine though). I thought this was problem with the whole program until I read this post and turned off the visualization. Now the MC9 is alot more pleasant to use (I was about to give up on it).
Also, on my machine at least, it doesn't seem to matter which visualization it is, even the simplest ones on the slowest settings will do it. Processor utilization is only at 2-4% for the program so I don't think it's a horsepower issue.
One more thing, I had been having trouble with MC locking up intermitently during burns (maybe 1/3 time), this too seems to have cleared up since turning off the visual.