You probably have a reception or interference issue. Try moving the Wii to within a few feet of the router strictly for testing purposes to see if that improves things.
Do a site survey and see what other APs in your vicinity are on what channels.
Make sure you don't have a 2.4GHz cordless phone. Or any of your neighbors.
If nothing works, wifi might not work for you and you will have to go wired.
Go to: www.nintendo.com
Click on: Support
Click on: Connecting to the Internet (Getting Online)