I think Knots2 is your solution. Others have reported success with TVersity. You may try a different UPNP server on your MythTV box. I used to use MediaTomb myself and disable the built-in UPNP server.

Also, the MythTV may not be doing "on the fly" transcoding to something that works well over the network and on lower power CPU like N900. So this must be taken into account.

The fact it plays fine locally tells me the N900 can handle it, but perhaps its network stack cannot. Try transcoding the file to something more basic or use an on the fly transcoding server like TVersity.