Network Cable is NOT unplugged Windows ARGH
Solution 1:
Is power-managment disabled on the network card ?
In my experience 99 of 100 times erratic network loss is caused by Windows putting the NIC to sleep from which it never wakes up again.
Go to the connection in "Network and Sharing center". Open the properties. Then hit the configure button just under the driver of the adapter.
Disable everything related to power-management. It will be on its own tab or be an option in the advanced settings.
If you can't disable it the driver doesn't support it in the first place so it is not the cause of your problem. In that case I'm inclined to put it down on some vague interaction between the network card and the switch. Updating the NIC firmware and the drivers (if possible) might help in that case.