I've got 4 AE's in my house (yeah, I know... I probably could've done without that many, but I actually DO have uses for all of them). One thing I really have noticed about the things is that they don't tolerate interference as well as many other wireless products do. Download a wireless scanner like macstumbler and see what other networks you have around you. Then pick a channel with the least amount of activity. NOTE: The way channels are setup in WIFI is kind of misleading. Channel 1 is not completely separate from channel 2 for example. They bleed together a bit. Think of the channels as the center frequency. The only real independent channels or each other are 1, 6, and 11. Think of it like FM radio. If you wanted to open up a new broadcasting station, you wouldn't pick (and probably wouldn't be allowed to pick by the FCC) channel 100.2 if some other station was already on 100.1 or 100.3.
Additionally, make sure that interference robustness is turned on in the AE (I believe it's an option when you click on the "WIreless Options" button). Take a note of any other devices you have that are 2.4 Ghz. My AEs hate my microwave for example.