Slow Airport Connection

I have an iMac (Intel 2.4 duo), an iMac G5 and 2 Macbooks + Airport Extreme. All Macs connect to Airport, can talk to each other and can access the internet. However...... the connection and data transfer between the Mac's and Airport is slow. If the Macs are plugged into Airport via network cable I get normal transfer rates (ping = 0.250ms approx) and connection speeds. When connected wirelessly connection and transfer rates are slow (x10 slower or more). All firmware is up to date on the Airport Extreme. I have tried turning the robust interference on and off. I have tried setting different channels. I have also done a hard reset of the Airport Extreme. I have tried a few different settings of the IPv6 but I do not understand this aspect so I am just guessing. Not sure where to go next. When I first got the airport several months ago everything was working well with great speed. The problem only seems to have happened over the last month or so.
Not sure where to turn next. Any help gratefully received.

Thanks
Phil.