distributer cap mods?

Too much vent may not be a good thing, may add to moisture on damp days. Engine heat should easily drive out normal moisture with standard vent.

A CDI ignition can easily burnout standard suppression wires. I had a set go from 5K Ohms, to meg-Ohms after a year.

Also about crank case ventilation, fumes can go past the distributor shaft bushings, if there is positive pressure. My first thought what to check for your problem. Is oil clean?