I am new to the forum ...

Have you verified their aren't any vacuum leaks since the intake manifold gasket work was done?

Get a can of carb spray, and while the machine is idling, spray around different places near the intake, and see if the idle is affected.

I am in the process of removing my lower intake manifold, and was reading on Alldata.com, that when reinstalling the lower intake with new gaskets ... they recommend a 1/4" bead of RTV sealant along the front and rear surfaces, and run it up .50 inch on each side. The RTV they recommend is Wacker ("vaucker") T-330 ( from memory ). They also note that the Wacker brand is more of an adhesive sealant, and is safe to use in the intake area, ( "do not use to seal antifreeze areas" was a warning too ).

 

the Carb cleaner isn't that great of a test (it atomizes very easily, and will float near the throttle body opening, causing a false idle fluctuation).

a better method to test for vacuum leaks, is to take a propane torch, and open it (but don't light it) and direct the nozzle all around the intake. between the intake and heads, base of TBI, sensor's, etc. if there is a vacuum leak, it'll show up right where the nozzle is pointed, giving you a more pinpoint location of a leak.