iMac Loses Internet Connection

My internet connection is through Ethernet and I lose my connection whenever I'm not on my computer for several hours (like overnight). My internet conenction from other computers, iPhone, ipad all work and the only way I can get back online is to re-boot my MAC. I'm running OSX 10.8 on a 3.2GHz Intel Core i3 with 12 GB memory.

Have you updated Mountain Lion to 10.8.1 yet? Also, you might check with your internet provider (ISP) and see if they have a time out set on your connection for periods of inactivity. Certain ISPs will automatically cut the feed based on periods of inactivity. Not all do, but some do. Generally with an ethernet connection you should have a connection 24/7 unless there's a hardware problem. If you have your ethernet connection setup thru a modem or modem - router combo, check that too to see if a time out is set.