Check for cracks on the two air hoses that go to the Idle Air Control Valve that is underneath the Air Filter housing (take off air filter housing) toward the front. Caused mine to to this until I replaced/repaired the one on the right side that had a crack at the neck. Also, check all your hoses for vacuum using a mighty vac type tool. Could have a little leak somewhere else but my bet os with the IACV hoses.

Steve