The problem is most likely your pressure tank. The rubber harbors bact. When you sanitize and the smell comes right back the most likely cause is the tank. Replace the tank with a fiberglass tank, Sta-rite has a good tank with a better bladder that does not harbor bact as much.
Also when sanitizing, it is better to sanitize the whole system not just the well.
To correctly sanitize you need to make sure that you have a pH less than 7 but more than 5 closer to 5 the better. anything over 7 is not doing you much good. Using 100 gallons of water with a pH of 5.5 with 2 gallons of 12.5% Sodium Hydrochlorite will give good results. Use Milk Stone remover from any farm store to lower the pH.
Water Treatment Specialist