How are you guys "twisting" the pairs? You should not do it by laying two wires next to each other, picking them up an the end, and rotating the pair together. What you want to do it hold one end in a vice or something similar and then hold one wire in each hand. Then, beginning at the vice, use the same motions that you would use if you were braiding three conductors. What you want to do is pass one conductor over the other. This method does not actually twist each individual conductor which messes with the structure of the strands, stresses the insulation, and probably disrupts the adhesion between the strands of wire and the insulation.