I've been fighting this for a while. There is something really badly broken with the MCP55 ethernet + linux. For some reason the PHY gets disabled, and then there's hell to pay trying to get it back.
See for example http://www.gossamer-threads.com/list...kernel/1046277
If you google around, you'll find lots of people with this problem.
I have several of the Asus M2N-SLI Deluxe boards; they're absolutely stable under load except for this ethernet issue. I have not found a reliable way to re-enable the NIC once the PHY is shut down.
It's a software issue but how does one force a reset?