Attic fans exhausting air with inadequate make-up air inlets depressurize the attic causing air to move from the house to the attic. Makeup air enters the home from outside increasing the cooling load. A 36" fan moves 1-2,000 cgic ft./min. 400 cubic ft. per min is a 1 to 2 ton of cooling load. Also check for A/C duct leakage. Try cooling without fan operating. On a very hot day, its OK for an A/C to operate all or most of the time. It is thought that an A/C sized close will provide much better humidity control.