[urq] another A/C - ventilation question

[djdawson2 at aol.com]

First, thanks to those that provided advice on my burning wiring smell... the advice led to a good fix.
 
The new questions...
Yesterday, I took off on my first trip in this "new to me" '83... driving from Denver to Gardiner, Montana.  It was about 100 degrees out while blasting along at quite a clip... A/C on full blast.  The problem... while driving along, suddenly the vents will switch from nice cold air, to hot outside air.  After a while, it became clear to me that if I simply lifted on the throttle, the vent would switch back to A/C.
 
After outside temps cooled, I was no longer using the A/C.  Using the vents for outside air, it soon became clear that they were switching sources as well... going from a nice heavy stream of outside air, to none at all.  On the fresh air ventilation side, however, I couldn't draw any correlation between throttle position, and air source through the vents.
 
My thoughts... clearly, the position of the flap in the air plenum is vacuum controlled.  I believe a vacuum is required to keep the flap in the "A/C" position (probably recirculation)... and for whatever reason, that vacuum is not maintained.  The loss of vacuum results in the flap switching to fresh air.  When I let off the throttle, I get the vacuum back, and it switches back to A/C.
 
So... any insight on how this works/should work?  There must be a check valve... where?  I assume I should check the diaphragm that moves the flap... and any plumbing.
 
Any BTDT's greatly appreciated.
 
Dave
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