My company's S-76 used to suffer pinhole marking on the rear edge of the stabiliser, which sounds like a similar problem to yours. It also used to suffer radio distortion in the cruise in certain wet weather conditions, which we put down to static build up.
It was allegedly lightning strike damage, although none of the pilots had reported this (I'm sure they would).
It has now been modded with static wicks on the rear of the stab. and the problem hasn't recurred since.