You mentioned these are Supermicro X7DBN boards. They use the Intel (ESB2/Gilgal) 82563EB Dual-Port Gigabit Ethernet Controller. There's an open bug here: https://bugzilla.redhat.com/show_bug.cgi?id=403121, "e1000: issues with Intel ESB2/Gilgal (82563EB)". It doesn't describe your problem, but complains about other issues with that NIC, and references related bugs.