Lost connection between airport extreme and macbook air when transferring large files

Can anyone help...?

I have an imac and macbook air wirelessly connected to a airpot extreme being used as a router with a hard drive for sharing files accross the network.

It connects ok but when we try and look at a number of large files one after the other, or copy files from the shared hard drive onto the macbook air, the macbook loses wireless connection?

A very strange issue which i cant figure out....

Can anyone help?

Thanks

Shirmy