As a regular on the BUF-ORD route I can almost guarantee that it is the problem in one or the other direction. AA has a real capacity problem since Eagle-izing this route, making this leg the weak link in many itineraries. Low fares disappear just as quickly as Milesaver awards. They're replacing several of the ERJ frequencies with CR7's, so a small improvement may be coming - unfortunately not in time for you.