sounds like the dealership you went to had some shady tree techs

 

Did you check the inlet line to the HVAC control for vacuum? Is it holding vacuum? Can you manually operate (and hold) each of the 3 vacuum motors (with a hand pump)? If you have all that, then its move likely the control head going out.. I think you are losing engine vacuum, most likely from the check valve under the hood. I'd start there