Unable to connect to wireless network

Strange problem started a few weeks ago.  I have 5 laptops that connect via wireless router, and my Latitude D620 is the only one having a problem.  For some crazy reason, after the laptop has been sitting for awhile, it loses the connection, and when I try to reconnect using the Intel PROSet/Wireless WIFI Connection Utility, I get "Unable to connect to wireless network", and the only way to get it back is to shut down/reboot.  Dell sent me a new WLAN card, and I put that in but it didn't solve the problem, and I uninstalled/reinstalled the Intel software, but still the problem continues.  Any ideas?  Thanks