I have a 27" iMac connected to my Airport Extreme through an Ethernet cable. I am running up-to-date Mountain Lion. Whenever I leave my iMac for a while, and it goes into sleep mode, it recovers as soon as I touch the trackpad. However, I am no longer connected to the Internet, and nothing seems to work except a restart. When I get disconnected that way, I can enable WiFi on the iMac and connect to the Internet without restarting the computer, but the Ethernet connection does not recover until I restart the machine. I did not experience this Ethernet disconnect problem until I updated to the new OS. Does this sound like a Mountain Lion problem? Any suggestions ?