Two things quickly came to mind as I read your discription
of the problem.
The first thing was the old foam seal is falling apart and some of it
might be preventing the blend door from closing completely.
The second thought was a cracked vacuum hose not allowing
the actuator to pull the blend door closed.
Try putting your ear next to the control head and shutting off the engine.
Listen for any hissing sound indicating a leak. If you don't hear anything
it means digging into the ductwork and watching the blend door and actuator
while someone works the controls.
I had a similar problem with my 97 Yukon SLE and it turned out to
be a small crack in a vacuum hose. As I was leaving on a road trip
south the next day, I just squeesed the hose to open the crack slightly and
rubbed
some silicone sealer into the crack as a "temporary" fix until I got back
home.
That was 3 years ago and it's still working fine.
Good Luck,
 JR