Hi Richard,
You are right, you have find somehow there. Some noise, or something in the way it's gets it power. Did you try different external powersupply's?
I use the TAI8586 1-Wire double optocoupled counter module without any additional (direct) power and more then 30 meters of cable in between. My module gets it power (parasite) from the 1-wire controller. Works without a problem.
Based on the Dallas Semiconductor DS2423 the TAI8586 1-Wire double counter module track two diferent series of events. MCT6 optocoupler isolates the 1-Wire network from the measured process. The module contains a 3v Lithum battery to count events even with the 1-Wire bus unconnected.