I had this same issue on the RC but am now getting it on the retail.
Windows 7 Pro x64
Asus P5N32-E SLI with nFocrce 680i chipset
On RC, if i installed the nForce driver
package then my ethernet would randomly fallover under high network activity (only internet would fall, LAN would remain) To fix i would right click network icon in system tray and select troubleshoot problems, this would then fix it.
But it did it so often that i ended up removing the nforce drivers
and letting windows 7 use its own driver, after which i never had a problem again.
Now i'm on retail im getting similar things although now its not happening with high network activity, it just happening randomly.
For retail i'm using the brand new nForce v15.51 drivers and its still randomly killing my network connection, but this time everything is falling over, LAN and net.
Is anyone else seeing this sort of behaviour when using the nvidia nForce driver package?
Its really irritating and i'm very suprised nVidia have still not fixed such a glaring bug.