Carefully inspect all of your vacuum hoses and PCV valve. Pinch off the vacuum hose to the power brake booster to see if the engine smooths out. If the engine smooths out, the Power Brake booster is the source of the leak, If no, then Check around the intake manifold gasket by spraying throttle body cleaner around the manifold with the engine idling. USE EXTREME CAUTION when doing this as the throttle body spray cleaner is highly flammable. Try to avoid spraying near the throttle boby as vapors drawn into the the engine through the throttle body will produce false results.If you had a noticeable change in engine speed, the manifold gasket has probably failed. Another possibility is the plenum pan gasket at the bottom of the manifold which requires removal of the intake to inspect and replace.