What are the causes of an internet connection goes on and off?

http://www.asus.com/Commercial_Servers_Workstations/Z9NAD6/

I put together a brand new PC w/ the above Asus motherboard that c/w 2 x NIC port. The software is Win 7 Pro, 32 bit, using as a workstation

The NIC was connected at my place about 5 times, for anti-virus update, windows update etc. and 5 out of 5, the moment I connect the RJ-45, the machine see the internet connection.

Bring this machine to the client's location, when the RJ-45 was plug in, there is no internet. I then plug that RJ-45 to the 2nd NIC port, again, no internet.

I then plug that same cable to my testbed sub-notebook, instantly I got internet. So it would looks like it's the machine. But that motherboard is very high end, and it works perfectly fine at my place.

Then I re-boot that PC, and as soon as I reboot, I got internet.

The next day, same time happens, when the RJ-45 cable is plug in, the machine is not seeing the internet, just a red X at the bottom right corner. But when I reboot, the PC sees the internet.

Almost as if there is some sort of time out.

I have seen something very similar to this a few yr. ago, and that's a voltage fluctuation at the router, once a UPS is connected to the router, I solved that problem

Does any1 else has any theory besides the above?

Have you tried releasing and renewing the ip?

No. I shouldn't need to. I have done this hundreds of times, not once needed to rel. and renew IP

Try turning off power saving in the ethernet adapters driver.

Swap out the cable. Even standardized things have tolerances. It is possible that cable's ends or the MB's jacks are just enough off to cause problems when combined with each other. Or that someone has spent years kicking or rolling a chair over the existing cable.
As a test, put a dumb switch between the client's network and the new PC. Some nics and some switches just don't play well together.

Update NIC Drivers and or install a PCI intel NIC

I'll dig out the power saving in the ethernet driver, I'll let you know next yr.