92 regal 3.8?

Does anyone know what would cause the motor to idle smoother in drive than in park? I was having a problem with the car idling choppy and stalling. Tonight I replaced the o2 sensor (service engine light showed a code 13 which is o2). Wasn't able to test drive it tonight but noticed it still idles a little choppy in park but if you put it in gear it smooths right out. Any ideas?

Try the throttle positioning sensor, it'll cause the idle to be kinda screwy