1996 Ford Falcon accleration problem

1996 Ford Falcon 6 cyl Two Wheel Drive Automatic

hi,
i have a ford falcon gli, 1996. when i press the accilrator and then release the accilrator but the speed remains same, it means that after releasing the accilrator pad the rpm does not go down it still stuck to 1500 rpm. then i shift my car neutal and then again i shift it into drive after that my car engine comes to ideal rpm.