Periodic problem with DNS resolution
This Apple support article, Non-responsive DNS server or invalid DNS configuration can cause long delay before webpages load, may help:


  DNS configuration
  
  This may indicate a non-responsive DNS server or an invalid DNS
  configuration. A DNS server converts a name to a numerical IP address,
  "www.apple.com" to "17.149.160.49", for example. You can connect to an
  IP address without any delay, because the need for DNS resolution is
  avoided. Here are four scenarios in which the symptom may occur:
  
  
  You have specified two or more DNS servers in the Network preference pane (possibly at the direction of a network administrator
  or your ISP), but the first server on the list has become
  non-responsive, or was entered incorrectly. The delay occurs while Mac
  OS X waits for the first server to respond, then it goes to the next
  server in your list.  Make sure the addresses are entered correctly.
  Changing the order of DNS addresses may improve performance. If your
  network administrator prefers that the primary server be used, you may
  wish to change back to the original order after the issue is resolved
  with the primary server.
  Your network may be configured to provide automatic discovery of DNS service, without needing to manually type the DNS server addresses
  at your computer. If you have manually typed a DNS address that is
  incorrect or out of date, your computer may wait for the incorrect
  address to respond (a "timeout") before succeeding at automatic
  discovery.
  You have not specified a DNS server, and automatic discovery of the service is not provided on your network. In this scenario, you can
  only connect via an IP address. Web pages do not load at all via a DNS
  name, such as is described in this article.  
  
  
  How to resolve
  
  Check with your network administrator or Internet Service Provider
  (ISP) to make sure your DNS information is correct. If your ISP only
  provides you with one DNS address, consider requesting a second one
  for use in the event that the first becomes unavailable.
  
  Once you have the address(es), you can use these steps to change your
  DNS information. Note: After using these steps, some Internet
  applications may need to be quit and reopened to return to normal
  behavior.
  
  Mac OS X v10.5 or later:
  
  
  From the Apple menu, choose System Preferences.
  From the View menu, choose Network.
  Click the interface you use to connect to the Internet, such as AirPort or Built-in Ethernet.
  If you use Built-in Ethernet: Type your DNS address(es) in the DNS Server field. If you have more than one, separate them with a comma.
  If you use AirPort, click Advanced, then click the DNS tab. For each server, click the "+" button under DNS Servers and enter the
  address provided by your ISP or network admin. Click OK when done.
  Click Apply.