Try putting these numbers in Network>TCP/IP>DNS Servers, for the Interface you connect with...
Then Apply. For 10.5/10.6 Network, highlight Interface>Advanced button>DNS tab>little + icon.
DNS Servers are a bit like Phone books where you look up a name and it gives you the phone number, in our case, you put in apple.com and it comes back with 188.8.131.52 behind the scenes.
These Servers have been patched to guard against DNS poisoning, and are faster/more reliable than most ISP's DNS Servers.