It happens to me at some times as well... For me, besides loading movies for half a minute or so, it also does not stream it fast enough from the PC, so it runs choppy.
You can:
-FTP the movie to the XBOX HD and see if the delay occurs too, if not it must be something related to the network
-play with the <cachesize>8192</cachesize> settings in config.xml
I haven't exactly pinpointed the problem, but one of the following helps (yes, it is network related in my case):
-reset router: i owe linksys WRT54G and most of the times it causes a problem: I just restart it and XBMP starts streaming from PC
-restart RelaX: relax in my case sometime goes into a wierd deadloop using almost all CPU
-delete XBMP settings (using original xbox dashboard and deleting files dorm E drive) and calibrate/set it again: don't jknow why, but this helps streaming running again
Regards.