I have had the same issue and have this workaround. But it would be better to install a new Nic since Intel has not developed a BIOS revision to solve the problem . I have contacted them and they admit to hearing of the issue but cannot duplicate the issue.
Intel DG41RQ Motherboard 775
LAN driver version 5.724 for Intel® Desktop 5/21/2009
Bios ver 0011 6/26/2009
****Workaround #1 ***
If Nic is not detecting
1.Turn system off and unplug the power cord.
2.Then Press and hold the Power switch for 5 seconds
This should discharge the board allow the Nic to redect in the BIOS upon repowering system
3.Go to device manager and change settings below.
4.Then change Nic Settings from auto-negotiation to 100mb or 1 Gig.
Can be changed to either 1Gb or 100mb
****Workaround #2 ***
IF Nic detects but cannot get online.
1.Go to device manager and change settings below.
2.Then change Nic Settings from auto-negotiation to 100mb or 1 Gig asabove screen shot shows
Well I discovered the issue causing my troubles. This problem was on my father-in-law's computer that I was redoing. He had a couple malware files that were rewriting the DNS as well as part of the Symantec Antivirus coding. So the DNS couldn't see the modem and the Symantec couldn't see these malware files to rid them from the system. I got them deleted and everything is fine now.