As you read in the XML, store it in an array. Using the random functions of Actionscript, pick a random movie URL and it's associated caption.
At this point you import the movie, attach it to a video object, begin streaming and don't start playing until the buffer is full.
The list can also be used with a combo box to make a select list (although with hundreds, another approach might be more practical.)
There are adequate samples and help docs within Flash to do all of the above.
