Make a New Location, Using network locations in Mac OS X ...
10.5, 10.6, 10.7 & 10.8…
System Preferences>Network, top of window>Locations>Edit Locations, little plus icon, give it a name.
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.
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...
(There may be better or faster DNS numbers in your area, but these should be a good test).
If that doesn't work try changing channels on your Router.
Also, turn off IPv6 if not in the EU, which may be required:
System Preferences » Network » AirPort » TCP/IP tab » Configure IPv6
Or whatever Interface you use.