a working or broken PTC resistor in between.--You should be able to notice if there is anything not the wire in that cable.
http://www.murata.com/products/catalog/pdf/r16e.pdf
Your MB once have a short or over-load so that over-current make the PTC create the 2V drop. For a good PTC, once the temprature drop to room temprature, the rsistor go back to 0. So don't be supperise if you meaused the ohm=0.
