First of all, it is quite obvious that higher the strength of class, greater will be chances of cheating. So, in order to prevent the students from cheating (at least consulting among themselves, let aside using books and on line directories), you may want to randomize the order of questions.
Remember, it is not a great idea to create several small sections within the question paper and randomize only the sub-sections, because it becomes easier for the students to find the sub-questions and discuss among themselves.
Therefore, you may consider randomizing even the order of sections as well as the order of questions appearing in those sub-sections.