Dropouts when accessing itunes library on an airdisk

PowerBook 1.67Ghz
Seagate FreeAgent 320Gb
Airport Extreme (version 802.11n)
Airport Express for airtunes

I have format shifted audio Cds to the Seagate (run as an airdisk) so that that I can wirelessly access audio tracks and then listen to them via airtunes though the Airport Express. The CD tracks were converted using apple lossless format and the Airport Express was added to the Airport extreme network through the Aiport Utility

The issue that I have is that there are frequent get dropouts in the audio as it streams from the airdisk (airtunes is not dropping out). I have used istumbler/airtrafficcontrol to establish that there are no other nearby wi-fi networks on the same channel as the airport network and to track signal strength – 50% with occasional noise readings of 10%. There are no active 2.4Ghz phones in this house when this problem is occurring.

When listening to tracks that have a smaller file size (3Mb mp3 tracks as opposed to 30Mb m4a tracks) this problem doesn’t seem to eventuate but the network can register speeds of 1.5Mb/s in Activity monitor. This is a small but frustrating and irritating problem – help.

Because the airtunes works fine from a CD my thoughts are that it is the access speed of the USB hard-drive or the speed of the Powerbook – can anyone give me some insight?