I think that ExSimGuy has it chaps
Gearoff - if you have a spare length of cable, strip back the insulation for about 100mm - you will see that the wires are twisted in discreet pairs. This is not obvious if you only strip enough for the plug termination.
The wire 'pairs' must be wired correctly - each pair forms a balanced transmission line to avoid cross-talk and noise pick-up.
If the pairs are mixed - cross talk is created - worse at high bandwidth.
It is impossible to check 'pairing' by doing a simple continuity check.
Good luck
AVPIN