I have the same problem. I'm normally using Linux, and have had a period of irratic network card behaviour, where I needed to unload/reload the network driver module, or reboot my machine to get networking to work again.
When my network card no longer is working after reboot, I tried windows, and here my NIC is no longer listed in the hardware list.

Any fw-upgrades I can perform?? Any help is appreciated!

Well I couldn't get it to work for a couple days, so I talked to Dell tech support and got myself new ethernet hardware.
...and a new motherboard because it's built in.

You may want to try the same. Thankfully I'm still under warranty, so I didn't have to pay.