Here's a kicker. Last night I loosened the foward 3 exhaust manifolds (I could loosen 1 bolt on the rear three). There was a definite improvement... the car idled at 16 psi with or w/o A/C on. Quick acceleration only produced a drop to 5 psi vice 0 psi with a rapid return and overshoot past 16 psi and then it settled back around 16-18 psi. Being a pessimistic pilot, I re-tightened the exhaust manfold and re-tested. Now it idled with steady vaccum at 10 psi w/o A/C and at 6 psi w/ A/C. Quick throttle advance produced a drop to 0 psi, moderate rise to about 15 psi, and then back to its "normal" 6-10 psi.
I fairly sure the exhaust system has a clog. So the question is: Why the change in symptoms from just loosening and then retightening the exhaust manifold? I can only suppose that the exhaust manifold gasket slipped, further restricting the exhaust.
Oh. With the air intake housing removed the car was very hard to start and coughed a bit before it would fire up. (this was before and after the above changes) The throttle body diaphragm will fully deflect manually and returns smoothly w/o binding.
By the way thanks for your help.