Hi MarkT,
I think that the problem is not the longitude of wires, because i supposed it was the problem, and i tried with a short piece of cable of only 15 cm... and the problem was still there in the same way that in a 1.5 meters long cable.
The cable have a ground line, what are near to all the others... i tried also connecting the shield of the cable to ground...
I will try to post here some pictures to show you everything...
Thanks for your interest!