 Vacuum leaks
Check first around the intake manifold for vacuum leaks - ANY major vacuum leak can cause this, even if the lines and valves which work the vent flaps are fine.
Frankly, this era car is prone to have ALL the original vacuum lines rotted or worn out. And there are a good many of them, too.
Before even starting the project I would check for cracked or decaying vacuum lines. At the first sign (which won't take long to find, I think) stop and go buy the 2 or 3 sizes of hose you are going to need to re-line the system. Do it one step at a time, and be thorough - and yes, those almost impossible to access lines buried deep and against the firewall ARE the most likely culprits. Check each valve or device as you come to it. Be prepared to replace them too.
After taking care of the lines under the hood, re-test your situation. Those lines are exposed and usually fail BEFORE the lines inside the car. You might get lucky and fix your problem without diving under/behind the dash.
In my case with my 88 LX, I had to remove the dash TWICE to get to all the bad lines, although the second time was mostly because of my own inexperience and lack of documenting all the wires, cables, and lines I disconnected along the way! (Take pics, before and as you go - and invest in a good manual, the expensive Ford document is what you will need).
__________________ tripleblack
"You can never be free until you let yourself go." | 
