I would suspect that the primary source of your problem is that a lot of your heat is just leaving your house through air leakage. The hot air is getting sucked right out of the house.
Many older houses were built with balloon framing, which at times create some heatloss problems, leaving unobstructed air flow, usually into your attic, then right outside through your vents. Although this situation is more frequent in older houses, I have also seen it in new houses where poor (Or on) techniques were used to eliminate air infiltration and exfiltration.
The solution: Look for how the air is leaving. Many professionals utilize fancy tools like "Blower Doors" & "Infrared Cameras", but common sense should lead you to the same places. Look for anything that is a hole in your thermal envelope, places where the air can move out. Things like: 1/ Balloon framed walls open into your attic 2/ Chimney chases that have not been flashed air tight at your ceiling line 3/ Insulation installed without an air movement barrier (Most insulation in and of itself does not stop air flow. If a hole is just "Stuffed with fiberglass, air will still move through it.) 4/ Dropped ceilings and soffits without a ceiling above (Many times batting is just suspended over such areas. The air beneath gets warm & convects through the insulation.)