We've got a fairly old A/c unit in our mobile home. Got it serviced at the beginning of the summer. Learned we can't set it at 74 and expect the coils not to freeze up. The unit worked great all Texas summer long. Even through the 20+ days of triple-digit heat. Usually had it set at 78. Now, the weather has cooled down, mid-70s, lower 80s. But now it has started freezing up again. . We change the filters about every 2 months, even though we keep a clean house and no smoking inside. What could be the culprit?