the following article tells how to troubleshoot self assigned IP issues:
this will more than likely resolve your issue or point you in the right direction for resolution!
I had the same issue. I resolved it by re-configuring my router. I added the MAC address of my apple computer to force the router to assign the same IP address to my computer.
It's not ideal, but it works, and remains fixed. It doesn't solve networks that you can't administer though, such as cafe wifi etc.