Like Fuzz said - Powerline connection as the first culprit.
The second culprit could be older ethernet card drivers, see if you can locate the latest drivers for the Ethernet cards on the two computers, update them and then try again.
Finally I would replace the two ethernet cables coming into the cables with new ones. I have seen many wonky cables causing problems.
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"In theory, there is no difference between theory and practice. But in practice, there is" — Jan Van De Snepscheu
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