My experience with this in the past has been windows cutting power off to the adapter:
Configuring power management using the user interface
To turn power management features on or off
Open Network and Sharing Center (click the Start button, type Network and Sharing in the Start Search box, and press Enter).
Click the Change adapter settings link in the upper left of the navigation pane.
Right click the network connection you want to enable/disable power management support on and click Properties .
Click Configure .
On the Power Management tab, check or clear the Allow the computer to turn off this device to save power check box.
When checked, power management is enabled on the network adapter.
When cleared, power management is disabled on the network adapter.