Vaccum leak around the HVAC controls, either leak from engine to controls, or after controller. Make sure the plastic vacuum lines under the hood are in good shape, there should be one to the heater diverter valve, and one to the HVAC Controller. If those are in good shape, there may be a leak in the hoses between controller and the blend doors. You can open glove box, shut slightly and push a metal clip on the left side then open it all the way open/down (Pull all your stuff out first!) And then you can see the different color vacuum hoses from the controller, maybe one is dry or cracked or just pulled loose. If that isn't it, there is a washer/grommet in the back of the controller where all those vacuum hoses go...I had to replace my controller because that grommet was too dried up in it and couldn't find a replacement.
