I have not had that happen with a shotgun, but have had it happen with a revolver. The firing pin was too long and was piercing the primers on every shell. I took a file to the firing pin and after each stroke of the file I dry fired the gun on an empty case that still had the fired primer in it. I continued until the firing pin no longer pierced the primer.
That your shotgun only does this intermittently makes me wonder if the problem is the firing pins or something else. If the firing pins are too long, it should pierce the primer every time or almost every tiime.
