The lan boards are not the same with the 6850. Before changing any parts, I would make sure that Windows OS doesnt have power saving features turned on. I have seen some OS updates that change the power scheme. I will assume Win 7 here and suggest that you navigate to control panel, then power options and make sure the PC has the sleep settings at never. Display off features are ok. I have pasted a graphic below. If these are ok, then you maybe having a problem with the NIC card in the computer or the lan card in the 6890N as you suspect.
If its a Lan issue, replacement part number for 6890N is G1530-62010 .
Thank you James, we will order the spare parts then we will check if it works.