It is important that the Ethernet interface is configured for the correct operation mode. Full-duplex is only available when a port on a switch is directly connected to the Ethernet port on the router.
In addition, both ends must be configured the same way. If an Ethernet interface is set to full-duplex and the Ethernet switch it connects to is set for half-duplex, the interface will operate but there will be a great many errors on the connections. This problem can be hard to troubleshoot if you have not encountered it in the field before. It is always a good idea to double-check all your interface configurations if you notice a high number of errors with low utilization of the interface.
For more information on Cisco Router Configuration and Troubleshooting, Second Edition, visit http://www.newriders.com/books/title.cfm?isbn=0735709998.