iMac's Wifi Connection Keeps Disconnecting When It Sleep & Restarts, OS X is 10.8.2

Hi. I'm on 10.8.2, the iMac (late 2009 i7. 8GB) keeps disconnecting after it sleeps or restarts, it see our router/wifi connection but will not automatically connect to it even though it's set to memorize that wifi connection. Is this 10.8.2 someone else is experiencing? It doesn't happen on my backup Snow Leopard OS (on an external USB drive). I've reset the pram and smc as well as repair disk and disk permissions.

Thank you & God bless.