Sounds like multipath reflections. Moving the base station a few metres might make a big difference.

At the moment my network is arse-about-face, with just my Rio Receiver plugged into the AP downstairs and the rest of the network connected via a PCI card in my desktop machine. This was because Ethernet-WiFi bridges cost twice the price of an AP at the time.

Now Ethernet bridges cost a more sensible amount I guess I should move my AP to a more central point and actually start using it "properly".