Can't connect to specified Airport network until powercycle of Base Station

For some reason after I boot my imac 17" 1.25GHz, if it doesn't connect to the wireless network, I select it from the drop down list and am prompted for my password. After entering it, it will give the unable to connect error.

If I powercycle the base station and try again, it works fine. It doesn't always do this, usually once a week or so. Could it be from being left on continuously, it locks up? Not a big deal, just would like to solve it.

Also, using latest Airport Extreme drivers and firmware.

Thanks!
-Ian