My temperature difference from the return and the registers vary. On cool days (and night) I get a 16 to 20 degree difference. During the middle of the day, the temperature difference drops to around 10 degrees (78 at the return and 68 at the registers). I'm hoping it could be as simple as a leak in the ducts coming from the return. These are in the attic where it would be cooler in the evenings and HOT during the day. Could something else cause this problem?