| Check your cabling connections for your network. We have cleaning people that will periodically snag the cables with a broom or vacuum cleaner.
Swap out your network cable that goes from your computer to the wall jack or test it with another computer that is working OK.
What do you mean by external NIC? How many network connections do you have on this computer? NEVER use more than one network connection per device unless it is going to different network. If it is a wireless connection, disable it. | 
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