I also made up all of the cables with the RJ45 plugs on, I did test them all before installing them but I guess that might not be a bad place to start looking for bad connections as well.
OK. Did you by any chance use the type of RJ45 that has a slot at the front that lets you pass the wires through then crimp then trim off the excess.

If so I have used them and chucked them out. Recently I made a 20Mtr patch cable (not cnc related) checked it with a cat5 tester that showed the errors. I did a continuity test pin to pin and it was fine.

But putting it into place it would not work. It turned out that the pins would not mate into the sockets properly.