Howdy. The built-in drivers for Ubuntu 9.10 had this same problem, but the 1.1.2 from Intel's site did as well. They appear to be the same as the 1.1.2 posted here. Tried upgrading to Ubuntu 10.04 beta since it has a newer kernel, those drivers do it too.
What will happen is that the network connectivity will suddenly cease to work, after which ifconfig will start racking up literally billions or trillions of errors on RX, TX, collisions, etc. There does not seem to be any pattern to it, and it doesn't require an extreme amount of traffic either -- last time I had received 1.5MB and sent 5MB before it died.
The NIC is Intel 82574L on Supermicro X8SIE-F (two NICs integrated onto the board).
What can I do to diagnose or fix this? This is an essentially unusable level of stability.