Windows 7: Randomly drops network connection

I hope somebody can shed some light on this matter as looking around the internet, there does not appear to be any confirmed answer to this problem.

A bit of background:-
I have a client who is has 30 machines that have been ok for the last 3 months.
That is, all except 1. This 1 particular machine would lose connection to the network randomly and sometimes running the Windows fix would fix the issue, other times, it did not. Even upon reboot it would not work sometimes.

This was then returned to myself for repair as we thought there maybe an issue with the motherboard NIC. So, the Motherboard was replaced but the problem still persisted.
Windows 7 Pro was reloaded a fresh and only the motherboard NIC drivers (latest available as of 5th June 2012) were installed. It was left to do Windows updates which completed ok. After a period of time, the yellow triangle appeared over the network icon identfying an issue. Again, running the windows fix on this seemed to fix it, but only temporarily as it seemed to come back after several reboots.
So rather than reboot, we left the machine on and it still came up with the same issue.
It seems to happen randomly not matter if the system is sitting there doing nothing or is being used to surf the internet.

After replacing all hardware and reinstalling Windows 7 on no less than 5 occasions, as well as running fixes recommended by Microsoft and various forums, the problem still pesists.

I have tried it on both our network and direct into the router, with the same problems occuring.

I would like to think it is an drivere issue, but that would not explain why the other 29 identical machines are not experiencing this issue.
If anybody has any recommendations, it would be greatly appreciated, as we are now tearing our hair out trying to fix this, but are getting nowhere.

Cheers,
Si

Additional: PC Spec might assist:-
Gigabyte GA-H61M-D2H-USB3, Intel H61, S 1155, DDR3, SATA II
Intel Core i3 2100, S1155, Sdy Bridge, 3.GHz, 5GT/s, HD2000
4GB kit (2GBx2) Corsair DDR3 PC3-10600 (1333MHz)
500GB Seagate ST3500413AS Barracuda 7200.12, SATA 6Gb/s, 720
Powercool PC750 PSU

Usual displayed windows error for network:-
"local Area Connection" does not have a valid IP Configuration.

This system is set to DHCP and addresses are being supplied without issue to other systems.

I have experiendced this also in the past. I dont know if this is suitable for your situation but have you tried setting the problem PC to have a static IP. In the past I have had similar problems and the only thing I could put it down to is that the router was assigning an address then the PC was losing connection. When the Pc requested a new IP address the router still registered it as having one so did not give it a new one. Setting a static IP address has always resolved this for me but as I said it may not suit your situation.

Thanks for the suggestion. Yes, I have tried various static addresses but to no avail. So damn frustrating this issue. So many people seem to have a similar issue, but it's usually a hardware fault that is the issue. Sadly, I dont feel that is the case in this situation.

If you can post the ipconfig/all from the problem machine it may help me to diagnose the problem.

I will get that posted as soon as it's finished reloading.....again

Here are the 2 images attached that show one system, that was built today and is currently set to DHCP and is downloading Windows Updates.
The 2nd shows the problem machine that is currently set to a static IP.

The long form ipconfig would be more helpful, where I can see all the other adaptors involved.

Should look like the picture below.

Additional Image. This is the problem machine, but set as DHCP. As I type this, it is currently working, but this WILL fail. It's 100% guaranteed.

Here is the long form.

Everything here looks normal other than for some reason there is no IPv6 address working or enabled with this connection.

Do you have IPv6 disabled for some reason?

Do you have the latest driver installed, I'm guessing Lenovo.
http://support.lenovo.com/en_US/down...DocID=DS019244