I am not so good on the electrics side but I guess it could be spark errosion for some reason. I have just been through some frustrating ignition problems myself and while I was at it I sorted out all of my 126 distributors and rebuilt the good ones. I found that the rotor arms when worn will rock about which could cause wear. I found that if the carbon brush was sticking then there was poor contact with the rotor arm. Then if the spring clips are faulty then the distributor cap will vibrate and if left will wear away the locating part on the distributor main body.