Strange networking issue Optiplex 790 MT

Hello all:

I'm having trouble resolving an issue on an Optiplex 790 MT. I don't have the system in front of me right now, so no service tag available yet as I already returned it to the owner thinking I had fixed it, but the NIC is an Intel 82579LM Gigabit chip, OS is Windows 7 Pro X64. The system does not have an on-board or expansion wireless card installed.

The situation is that the system won't recognize that an ethernet cable is plugged in. I received the device earlier today, checked the IP configuration, uninstalled the device and reinstalled all the latest drivers. I also checked setup to insure the device was enabled. Additionally, I ran the system diagnostics utility included with the system (F5) as well as the Intel Device Manager Diagnostics Utility provided on the support page. No problems detected with the hardware, but no recognition of the Ethernet cable, either. Lastly, I reset CMOS, still no dice.

I considered malware as a possible cause, but the system is not showing any other signs of an infection - no performance impact that I can see and system resource use seems normal, no pop-ups or other strangeness.

So I replaced the board, fired up the system, set BIOS to defaults and the system booted and immediately connected to the internet. I tested functionality to a number of sites.

I returned the system to the owner and she just contacted me saying that once again, it won't recognize the Ethernet connection.

So now I'm thinking maybe it is malware, or some program compatibility or firewall issue.

Any ideas?

Thank you.