surpised they didn't tell you it was a windows /router issue and you needed a mac /airport.....
bonjour protocol and their servers will be the two most likely problems, run a wireshark capture whilst your doing a download and see what priority your music files are getting, if you find its in the wrong category then ask plusnet to fix it, although as I said its likely just their servers not giving the music out fast enough
just because your paranoid doesn't mean they aren't out to get you
