Pinch off the vacuum hoses that originate on the engine while it's running. If the engine speed comes down, follow that hose to where it branches off and pinch those branches. Keep following the one that lowers engine speed until you find the leak.
If it's not a hose that's leaking, check the intake manifold gasket by spraying water around it while the engine is running and still cold. Look for any place the water gets sucked in and affects how the engine runs.