How many devices are connecting? Some routers have a limit.
Is any device using a Static IP instead of DHCP?
Though all of these steps may or may not be needed, I'm including them all.
Make a New Location, Using network locations in Mac OS X ...
http://support.apple.com/kb/HT2712
10.5, 10.6, 10.7 & 10.8…
System Preferences>Network, top of window>Locations>Edit Locations, little plus icon, give it a name.
10.5.x/10.6.x/10.7.x/10.8.x instructions...
System Preferences>Network, click on the little gear at the bottom next to the + & - icons, (unlock lock first if locked), choose Set Service Order.
The interface that connects to the Internet should be dragged to the top of the list.
10.4 instructions...
Is that Interface dragged to the top of Network>Show:>Network Port Configurations.
If using Wifi/Airport...
Instead of joining your Network from the list, click the WiFi icon at the top, and click join other network. Fill in everything as needed.
For 10.5/10.6/10.7/10.8, System Preferences>Network, unlock the lock if need be, highlight the Interface you use to connect to Internet, click on the advanced button, click on the DNS tab, click on the little plus icon, then add these numbers...
208.67.222.222
208.67.220.220
(There may be better or faster DNS numbers in your area, but these should be a good test).
Click OK.
If that doesn't work try changing channels on your Router.
http://x704.net/bbs/viewtopic.php?f=17&t=6291&p=76975#p76975
Also, turn off IPv6:
System Preferences » Network » AirPort » TCP/IP tab » Configure IPv6
Or whatever Interface you use.
