You can use the problem set version number for this purpose. That is different for each student and assignment, but is the same within a given assignment. You then use that to seed each problem by adding SRAND($psvn) to the beginning of each problem.
Sorry to be rather late on this topic. Is it possible to have a sets seed for another set in the same course or to use anothers set seed? Thus it would be possible to repeat the same question with multiple choice questions out of an array in another set without repeating the exact same one.