Try delete the current wireless connection and password info on the MacBook and then start over again and setup a new connection.
Open System Preferences (gear icon on the dock)
Click on WiFi on the left
Click Advanced at the lower right
Locate the listing for your wireless network name and click on it to highlight it
Click the - (minus) button to delete it
Click Done at the lower right
Click Apply at the lower right
Open Macintosh HD > Applications > Utilties > KeyChain Access
Locate the listing with the wireless network name and click on it to highlight it
Delete that listing
Restart the Mac and set up a new connection again
My Mac would connect at some locations, but not others. I did not think it was a wireless card issue on my Mac either, but a new card fixed the issues.
You still need to let the guys at the Apple Store take a look and run diagnostic softwere on the Mac. Not much else you can do.
A pure shot in the dark would be to reset your wireless router back to factory default settings, then reconfigure it again. Might be worth a try if you have the time. Good luck.