When I press apples and crabapples, I use sulfite (campden) at pressing. That keeps the wild yeast and bacteria at bay. Then I add my chosen yeast strain the next day.
I don't pasteurize by heating, as that changes the flavor of the juice as well as sets the pectins.
You could try a double dose of sulfite, and see if that helps. Also, make sure that after primary you rack to a vessel with NO headspace, within two inches or so of the bung, and make sure it's filled to the most narrow part of the vessel. That would really help prevent mold and other oxygen-loving microbes from taking hold.
