Les, have you ever checked the plastic vacuum line under the battery? I have had problems with my car every so often, for over a year. I would check/tighten the plenums, replaced the throttle body rubber gasket, replaced clamps on Y-pipe, checked all known vacuum leaks. Replacing parts may have solved some of my problems at the time but I would still get hicups intermittenly, mostly after I had the AC/heater on. I saw the thread about that line under the battery and decided to check it. It was cracked a couple of places so I repaired/replaced it and have not had a proble since.

Now your problem sounds like a bigger leak than just some pressure from a small line but it may worth checking if you haven't yet.