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Could this be caused by a bad intake manifold gasket?

I would say a bad intake manifold gasket is not the only possible cause, but for that vintage of Isuzu, it is the most common cause.

Have you tried the flammable aerosol test? (engine running, spray carb cleaner or similar aerosol around the perimeter of the intake manifold, if the cleaner is drawn into the engine through a leak the rpm will change.) Not a foolproof test, but a cheap one...

Steve Carlson - 95 Trooper LS expo rig
Serenity now!