Airport Express throws users of network as well as itself

Hi, I have just purchased a new AE. Took ages to set up as I kept getting errors, however, after some research I discovered this was down to me using wep encryption. This is now updated and the AE can join the network. However, whenever I turn it on to use Airplay (the only reason I purchased it) connected to my HIFI it gets through a minute or so of a track and then disconnetcs and, at the same time, throws everyone else off the network, those devices then have to all reconnect.

The AE is definately set as JOIN (not extend)

I have tried changing the channel number on the WIFI router

The AE has the correct encryption info

Can you think of any other settings that could be causing this conflict. Hopefully something can cure it otherwise it will be going back for a refund.

cheers