Airport interference issues
Often problems with connectivity on wireless networks can be down to interference from other wireless networks and devices such as baby monitors, microwaves and notably cordless phones.
This is because other wireless networks and these devices use the same 2.4GHz frequency that Airport (and other 802.11b/g wireless networks) use.
If it is a neighbouring wireless network, find out what channel the neighbour's network is on using the Airport Traffic Control Widget and ensure your channel is at least three channels away.
If it is a cordless phone then try your network on channels 1 or 11 as most cordless phones use channel 6 (but not all).
You might want to replace your cordless 2.4GHz phone with a 5.8GHz phone.
Another option which is sometimes the only option in a crowded 802.11b/g wireless airspace is to change to 802.11a. Most recent Macs can use 802.11a, and 802.11a is also an option on the new Airport Extreme Base Station and 802.11a wireless bridges are also available. But this is an extreme option which is only recommended in a crowded wireless environment.