Two possibilities come to mind:
A vacuum leak between the mass air sensor (MAS) and the intake manifold or a split vacuum hose that's attached (or should be) to the intake manifold. As the motor slows a small vacuum leak becomes proportionately greater to the air moving into the motor. Unmetered (leaking) air dilutes the air / fuel ratio so the motor will be underfueled and stumble. Idle the hot engine and listen for a hissing sound that indicates a vacuum leak. Check the clamp connections to the MAS and those rubber boots, also check the air duct's clamp to the intake manifold.
The second is the fuel pressure regulator, a small drum attached to the fuel rail to which the fuel injectors are attached. The regulator has a vacuum hose attached to it (hose's other end is attached to intake manifold). With engine OFF simply remove the rubber vacuum hose from the regulator; the regulator is defective if fuel drips from it.