Is there something in the house running on the wireless network using 802.11b? That might be slowing things down enough to not allow a smooth stream to go to the Airport units. They do use a bit of bandwidth for the music, as Apple chose to do the audio decoding on the PC, then stream an Apple Lossless audio feed to the units. It's how they allow the music to be synchronized across multiple Airport units and a PC.

I can't comment much on the wireless performance of them though, because when I used the two that I have in my old house, both were wired via ethernet, then used as access points at both sides of the house. Any music streaming at most would be wireless to one unit, then ethernet to the other.