You can either manually change the config file or change the settings within the gui.. Personally I think doing it manually is easier
Exit xine-ui.
Edit ~/.xine/config
Find and change the line
#engine.buffers.video_num_buffers:500
to
engine.buffers.video_num_buffers:900
(don't forget to remove the '#' from the start of the line)
and restart xine.
You may have to play with the value a little.