Sure sounds like an intake manifold gasket leak, having experienced this problem twice. In any case, it's easy to check - with the engine running, spray carb cleaner or similar flammable aerosol around the margin of the intake manifold. If the cleaner gets sucked into the manifold via a gasket leak, there will be a sudden change in rpm. Sometimes you can even hear the leak hissing.

And it's a common problem on that vintage 'Zu.

Steve Carlson - 95 Trooper LS expo rig
Serenity now!