To open Device Manager in Win8:
1. Swipe in from the right edge of the screen, then tap Search. (If you're using a mouse, point to the bottom-right corner of the screen, move the mouse pointer up, then click Search.)
2. Enter 'Device Manager' in the search box, and tap or click Device Manager.
When in Device Manager, look for Network Adapters and look for your wireless adapter, then right click on the adapter name and select 'properties'. Then go to the Power Management tab and untick 'Allow the computer to turn off this device to save power'.
As your snag is only occurring when the computer is on battery, perhaps the system is being overly sensitive and tries to save power too soon?