Everything was good, but I had an extra 8Gb RAM installed, taking it up to 16Gb (8Gb x 2)
As soon I turned it on, the issue was back.
Update driver. Already has the newest driver.
Network Reset, as above.
In Device manager, go to Ethernet properties and unclick 'Allow computer to turn off'.
And it's still the exact same.
Every minute the connection turns off for 10 seconds or so, then connects again. Another PC connected to the same router has no such issue (both cable connections have been swapped)
One difference shown is the ethernet resource settings, the 2nd RAM stick shows as this.
Right or left mouse buttons doesn't do anything to it.
Any ideas?