For those also having problems - just to let you know iworked out how to fix
it - I used Windows Media Stream Editor to demux the audio and video and
remux them back into one file - plays flawlessly on the pc and the x360 -
can only imagine the header information got corrupted
It's a utility included with Windows Media Encoder:
http://www.microsoft.com/windows/windowsmedia/forpros/encoder/default.mspx
Details here:
http://www.microsoft.com/windows/windowsmedia/howto/articles/IntroEncoding.aspx
under "Additional features with encoder utilities."