They are literally installing the cold air return as I type this. New furnace is in. Sent the kids out of the house as I wasn't sure how long this would take.That sounds like a major source of your problem right there! Keep in mind that round ductwork has far fewer square inches then square, and cold air returns can use a beam space that does not need to be encased in sheet metal on all sides. Cold air returns actually need to be larger in volume then the hot air ducts, because your furnace can push air more efficiently then it can draw air, the filter is also on the cold air side so you want as little restriction there as possible.