I had a similar problem like this on a switched network.
Check the folowing.
a) verify that you are using the VENDORS driver and not MS.
b) Verify properties of the NIC. Make sure the speed, duplex and flow control are properly set.
c) Verify the properties of the switch you are connected to. Some switches even though they are set to Auto will not properly handle the connectivity of the NIC. You will need to force the switch, Have seen this with Intel NICS and HP switches 4100 series(cisco IOS) as well as HP 9100 series (Foundry with cisco IOS). 3Coms with flow contol off will do this.