Hi,
If the spots aren't caused by dust on the films pre-exposure, it's possible that air bubbles are sticking to the films during development. You can prevent this by pre-soaking your films before development using water at or slightly below the development temperature with a couple of drops of wetting agent added. This will evenly soak the film and swell the emulsion for development. Leave it in the tank for a couple of minutes, then drain before pouring in your developer.
You can also add wetting agent to the dev, but again only a couple of drops.
I use this technique with 120 and 35mm films, and rarely see a pinhole.