Couple of things to try...
Try putting these numbers in Network>TCP/IP>DNS Servers, for the Interface you connect with...
For 10.5/10.6, 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...
Also, turn off IPv6:
System Preferences » Network » AirPort » TCP/IP tab » Configure IPv6
Or whatever interface you connect with.