Go to /Library/Preferences/System Configuration
Move all the files from the System Configuration folder to the Trash.
Restart your MacBook.
I wouldn't bother with a used airport card. Might invite more problems.
If deleting those files didn't help, try another channel.
Open AirPort Utility /Applications/Utilities
Select your base station on the left.
Click: Manual Setup
Select AirPort from the menu then select the Wireless tab.
Click the pop up menu next to: Channel. Try Automatic. If that doesn't help, try different channels.
Make sure and click Update as soon as you make any changes. It may take the router a minute or two to reset itself.
You can check for available software updates from your Apple menu top left in your screen.