Hi Larry

You may want to double check the vacuum elements for potential leaks, easily accomplished at the switchover valve assembly behind the glove box. (<20 minutes) As you test the elements for leaks try not to exceed 3.5-4.0 inHg during the test.

One other thought, when the A/C system is turned off are you able to feel air passing through the ventilation system while driving down the road? If so you might have a Fresh Air/Recirc element that is contributing to the reduced efficiency at the vents.