You probably have a reception/interference issue.
Try changing your router to a different channel (1/6/11). Many wireless cards include a site survey feature so you can scan and see what APs are out there using what channels, that way you'd know which has the least interference.
Your router could just have too weak of an antenna to get to the room with your Wii. Not all routers are equal, and you get what you pay for. Upgrading to a better router with stronger transmit power would help. Some allow external antenna connections so you can hook up an even better antenna on top of that.
More help can be found...
Go to:
www.nintendo.com
Click on:
Support
Click on:
Connecting to the Internet (Getting Online)
Also see:
http://www.nintendo.com/consumer/rou...outerIndex.jsp