I would start by pulling down a little bit of soffit to see what your dealing with. You should have a 1" air space between you insulation and your sheathing. if you don't have that then you will need to install some foam baffling (home improvement store). Add more vented soffits in the areas where you have none. Then provide an exhaust fan (or maybe two - one at each end) close to the ridge. Considering you extreme temperatures in you attic I would make it a powered ventilator with a thermostat/humistat. You can buy them at your local home improvement store.
I am assuming you have fiberglass batt. insulation? If so, you can make it much more efficient by adding an air barrier over the surface exposed to the attic. Moving air through the batting greatly reduces its effectiveness. Tyvek or equal is a great fabric type air barrier.
I am assuming you have fiberglass batt. insulation? If so, you can make it much more efficient by adding an air barrier over the surface exposed to the attic. Moving air through the batting greatly reduces its effectiveness. Tyvek or equal is a great fabric type air barrier.