Even though I dive regularly, I always used to get sore gums from my regulator. This inflammation occurred irrespective of whatever second stage I happened to be using. I then tried out a moulded mouthpiece, and have never looked back. You soak the 'blank' mouthpiece in boiling water for 30 seconds, then bite on it to create an impression of your own teeth. I know that some divers dislike the large size of these mouthpieces, but for me they represented the end of an annoying problem.
Custom Divers, £17.50
www.customdivers.com