After severals days of banging head against wall, one of our network technicians discovered that Dell's PowerEdge 2950 switches eth0 and eth1 under Linux. *sigh*
Fortunately, Dell has a solution or workaround for this problem that occurs with the on-board Broadcom NetXtreme II 5708 GigabitEthernet NICs. We have also found that after prolonged periods of inactivity, the Broadcom NIC seem to go down as if in powersave mode or so. I can't however confirm this issue, as we're still resolving and confirming it.