I can confirm this exact issue: It's not a simple connectivity issue. The ethernet link does not change state; server is reachable via ping; ssh connects flawlessly occasionally, then seemingly randomly does not connect or existing ssh session drops. This occurs on Ubuntu 10.04 and 11.04. Following hheimbuerger's suggestion I gave the server a static IP, this seemed to fix it.
Workaround: Change adapter from DHCP to static.
