The last time I used Plus-X was in the 1980's--it was 120 Plus-X Pro, and I got pinholes. I quit using Plus-X because of it, as I did not get pinholes with Tri-X or Verichrome Pan.
Perhaps it was technique, perhaps not. But I no longer use an acid stop bath, and prefer, instead, a water rinse. I use an alkaline fixer. I suggest you try using a water rinse instead of stop bath, at least as a test, and see if that cures the problem.