iTunes/AirPlay dropping out

I have an Airport Extreme (set to b/g/n mode) wired into an ethernet network, and a separate Airport Express (set to b/g mode) that joins the network through the Airport Extreme. I've checked and signal strength seems to be good, response time for pinging the Express through the Extreme is 1-6 ms, and I've set signal robustness on. Despite this, and despite efforts to switch channels to ensure that there's no significant interference, iTunes keeps cutting out when playing music through AirPlay. When it cuts out, and I try to re-start the music, I get the following message:

The AirPlay devices "Express2" cannot be found on your network. Please verify your network settings and AirPlay configuration or select another device from the menu at the bottom of the iTunes window.

This comes up despite the fact that connectivity remains good to the Airport Express device. I've tried upgrading and downgrading the firmware on the Airport Extreme, and it seems to make no difference. To make this problem even more confusing, there are some days when the setup works without a hitch, and other days when it can't play more than 30 seconds worth of music without failing repeatedly.

In terms of software, I'm running Lion (this has been happening under 10.7.1, 10.7.2 and 10.7.3) and iTunes (this has been happening under 10.5.1, 10.5.2 and 10.5.3).

The only thing I can think of is that iTunes AirPlay implementation is timing out for some odd reason, but I can't seem to find any meaningful settings for AirPlay in iTunes.

Does anyone have any suggestions for adjusting the time-out in iTunes or other steps to take to narrow down the problem?