What we need first to recognize is that the primary way for heat to move is through convection, or air movement. If an air-tight seperation is not made between the living space & the structure, especially the attic, air will just move past any insulation & carry the heat with it.
The first thing to do is get rid of the power vents! They create a great negative pressure in the attic, drawing more air into the attic & more heat along with it (It mayappear that the attic is cooler because with the volume of air moved, more outside air is drawn into the attic dilluting the hot air from the house. The hot air is still leaving along with your heating money. The net step is to seal any air leaks. In plugging them, fiberglass does't count. It only filters the air as it goes through. It must be a material or method that stops air movement, such as styrofoam pieces cut to fit & caulked (dense packed cellulose also virtually elliminates air movement, but it must be DENSE PACKED.) As you have described your house, check to see if the floor under you upper level is open into the kneewall areas. Also the walls into the dormers.