I have tried many permutations: different settings on the AirPort Express, different configurations of Ethernet cables and Wifi, you name it. In every case, there are at least occasional patches of silence lasting 0.5-5 seconds (playback does not pause during this silence). My findings: If the AirPort Express is not plugged into the Ethernet, but instead forced to use the Wifi coming out of my AirPort Extreme nearby, then the glitches are almost constant. It is so terrible, during five minutes of "playback" there might be thirty seconds of actual sound. If the AirPort Express is plugged into the Ethernet, it doesn't matter whether my MacBook Pro is plugged in. There will be occasional glitches. At least some of these glitches are coming from the MacBook-end, because they sometimes coincide with Time Machine backups or processor-intensive tasks. What the hell? Is AirTunes just bad? Or is it my MacBook Pro, is it overloaded or does it have some errant process stepping on the AirTunes' toes?