Hello Muirdach
Do you have the latest software and firmware updates for the Airport Extreme Base station? You can check by going into the Utilities folder and opening Airport Utility. In the system bar, click Airport Utility and check for software updates. I believe it should check for firmware updates as well.
If you have the latest updates, give
this post a shot. I just created it yesterday after having a very unreliable wireless connection. I realize it isn't the same symptoms you are having, but the idea is still the same.
Let us know if you need further assistance.
Best of luck
Searay