How exactly do you have your library structured? Is it just media on the NAS or the whole iTunes collection including media files, library files, artwork, etc?
iTunes does everything through a central database file (library.itl). This stores the information as to where your media files are located. If for any reason you start iTunes and it cannot find the media in the location exactly as listed in the database file you get broken links. A common thing to happen is starting iTunes before the external drive is turned on. Sleep can cause issues, as can anything that changes the identity of that drive as iTunes expects to identify it when iTunes first starts up.
From reading posts here I'd say in general that seeing issues with NAS and iTunes is not too uncommon and the situation where the glut of media outgrowing internal drive storage on the small computers people want these days is still outpacing the ability of iTunes to truly handle network storage that this glut is requiring.
What are the iTunes library files? - http://support.apple.com/kb/HT1660
More on iTunes library files and what they do - http://en.wikipedia.org/wiki/ITunes#Media_management
What are all those iTunes files? - http://www.macworld.com/article/139974/2009/04/itunes_files.html
