Well, the majority wins. I reverted back to the 60s' and used the old "tube in the ear" trick. Sure enough, the intake manifold gasket is leaking in two spots on the right bank of cylinders. You can actually hear the hissing noise without the rubber tubing but I plan on shoving a tube in the Isuzu service manager's ear so that he will have no doubts. Hopefully that will cure the problem. Only have about 500 miles left on warranty.

Triathlete is correct about the misfiring. At low idle speed a vacuum leak is very noticeable. At higher revs, the amount of air thru the leak is small in relation to the total air intake.

Helps to have a 1970s' British sports car (1971 Marcos). They all leak something! But it's fast!