You need to go to Device Manager in control panel\system\hardware\device manager then check on things like battery, I had this problem with my HP Laptop I think it comes under Networking to Shut Down when running on battery, you need to disable this feature for it to keep running. If you plug the Mains Adapter in to Prove my Point it should not turn off.
Found it, under Wireless Networking in Device Manager right click on it then Properties\Advanced\Power Management in the Drop Down Box, the slider wants pushing right across to the right to full. It will most likely be set to Low
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