When you are connected remotely into corpnet, there is always going to be spuradic traffic, so that is why it won't shutdown the NIC.
Can I make a suggestion and throw in a NIC? There is one 1x PCIe slot in there, so if it is free, I would consider throwing in a GB NIC and disabling the onboard 10/100NIC. If only for testing purposes, it may prove useful.
If that isn't possible, check the Hardware & Sounds -- Power Options in Control Panel and look at your plan settings (which to test should be set to High Performance)
The driver is obvously suspect, so keep an eye on the Intel site for updating drivers for the chipset, especially as we get closer to the Windows 7 release date.
Hope this helps, a bit...