I doubt that it is the intake manifold gasket, but replace your intake manifold gasket anyway because by now it is as hard as a rock. ANYTHING that disturbs it (e.g. motor mount work, transmission work) will generate a leak.

A couple of the nuts are tedius to loosen and tighten, but replacing that gasket is an easy 20 minute job if it all comes out in one piece like mine did.

Even if you don't replace it, include one in your next parts order so you will have it on hand when the inevitable leak occurs.

Check all your vacuum hoses for splits occasionally. Hose failures really don't seem to bother the car much, but they exacerbate any other problem you may have with the idle.

As noted elsewhere, the way to look for a vacuum leak is to spray a flammable liquid on everything with vacuum inside. When the car accelerates or smooths out, you've found the leak. I've heard that a propane torch works well for this also, but you would have to be able to get the tip right next to the leak. Remember that chlorinated solvents, like carb and brake cleaner, are not very flamable, so they are safer to use but not as effective at finding the leak. I use starting ether with a cap and spray tube off from a can of cleaner.
It works great but I'll probably burn my beard off someday.