Could be the network card is going into power saving mode.
Goto Start, Settings, Network and Dialup Connections, double click on Local Area Connection, and click Properties. You should now have before you the Local Area Connection Properties box showing the revelant Ethernet Adaptor. Click on the Configuration button, click the Power Management tab and untick the "Allow the computer to turn off this device to save power".
That's often the problem...
ROGER - G0AOZ.