the temp diff sounds right it is usally about 10 to 15 degrees difference.it sounds to me like you have a problem with your insulation or something letting your air escape or allowing the heat get in and also where is your thermostat located. it should be in a central spot or close to the return air to get atrue reading and make sure its not in a spot where the sun is on it.also do you have a return at the highest point in the house that would make a difference.