Make the marinade: Place the lemon-grass, garlic, ginger, peppers, and cilantro in a mixing bowl. Stir in the sugar, pepper, soy sauce, fish sauce, lemon juice, and oil. Pour this mixture over the ribs, rubbing it onto the meat on both sides. Marinate, covered, in the refrigerator for at least 6 hours or as long as 12. Turn the ribs two or three times while marinating.