I didn't have time to read all your posts, but has the intake manifold ever been removed? If so it's real easy to "missalign" the intake, when re-installing. I've done this a couple of times while working on an old stovebolt. The alignment rings can hang up in the port and cause a major vacuum leak! I can relate to your frustration, I've been known to make the same mistake over and over? Maybe you can get another mechanic to take a look, get a fresh perspective? Good luck!
