Make a New Location, Using network locations in Mac OS X ...
The Interface that connects to the Internet, needs to be drug to the top of System Preferences>Network>Show:>Network Port Configurations and checked ON.
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.
You haven't provided much information but where I'd start is by resetting the modem & router, then from there check to see if you are competing for channel bandwith with your neighbors. Download and install iStumbler (Google it) and check to see what channel you and the neighbors are using, let it run for 30 minutes or so. If you find you're all using the same channel then change it. If you aren't competing for channel bandwidth then reset your DNS settings from the defaults your ISP provides. You can do so by:
1. System preferences
3. Click Advanced
4. Click DNS tab
5. Click + sign
6. Add 188.8.131.52
7. Click + sign
8. Add 184.108.40.206
9. Click OK.
10. Exit System Preferences