I'm also having problem with playing mp4 files in MC (I've been reporting these issues in the main build thread.)
I am using the latest versions of MC, and playback of the SAME files is fine if I play them in QT.
The problems are especially bad when playing over Media Server, but the issues are NOT just a network issue since I can play the same files over the same network in QT without problems. Normally, mp4 files won't play at all over Media Server. I have discovered, however, that when I shorten the filename and path, they WILL open and start playing, but not very well. There is a lot of starting/stopping/skipping, etc. When I use my standard name and path, however, they don't play at ALL over Media Server. I have NO problems playing them with Quicktime, both locally and over the network (using the longer version of the path/filename.)
Thanks,
Larry