This sounds very suspicious of either a wifi LAN bandwidth issue or wifi interference.
I used to get stuttering playing from macs to Express until I changed router (or perhaps it was teh channel settings) - microwave was a common cause of interference.
Current Express maxes out at CD quality 16 bit 44.1 kHz.
AppleTV2 apparently upsamples to 16 bit 48 kHz fro some odd reason.
I don't think we know enough about Airplay internal working to know if it preserves audio quality or adapts to lower speeds by downgrading quality. In theory I would expect it to preserve CD quality from lossless rips but there are many factors at play.
An option I would consider would be hooking up the Macbook to the reciever via optical/Toslink to miniToslink, then using the iPad Remote app to control itunes on the Macbook.
Add something like BitPerfect app, Fidelia or Audirvarna app into the eqaution to improve the sound quality if uisng analogue out instead. Not tried to see if it improves optical jitter.