Sounds like a Vacumn leak to me. Make a good visual - CK PVC at valve & rear of Plenum also EGR connection at back of Plenum also Vacumn line at throttle body and at fuel regulator (Fuel rails Driver side rear).If all else fails spray Carb cleaner with engine running at intake manifold gasket mating services if RPM rises Try tightening manifold, if that fails new Intake Gaskets req. order
PN 8-97237-538-0 two req at $18 + each - 1-800- 727-8066 ask for Merlin. S&H is free

Torque value is 13 FT LBS on all intake manifold nuts & Cap Screws.