How large is your iTunes library? I have heard of some people running into problems with libraries exceeding 20gb. I suspect that there may be some kind of fragmentation issue. If you have a spare computer, you could try copying it there and see if the same problem happens. Also, make a backup of the library files and then try having iTunes rebuild the catalog.
I can't stand how clunky iTunes can become. For me, I don't have any audio stopping issue but it takes almost a minute to start up before I can start using it. I don't have any issues like this with any other software on my laptop (Windows Vista Ultimate).