I have buick lesabre 93,has high idle that comes and goes, mech connected handheld computer, it said it was..

like i was saying the diagnostic computer said it was the oxygen sensor, i replaced it right away but still the same problem. when i start the car it seems like idle is extremely high, after i pull out and drive for a mile or so and stop at light i turn off car and turn back on its calmer but still not all the way calm, i do that a couple of times and everytime i do it it seems to calm down a lil more. sometime i give it a real good revvv and it calms down all the way! and ill drive around for awhile and it acts perfectly quiet and smooth, but once i park and turn off car when i turn back on like 10 min later the idle is high again, not so severe like when in the morning but still high. by the way when i first turn on car it will speed up to 40 mph on its own! until i turn off then back on. does anybody have any ideas? tired of not knowing whats wrong with this car. please help!!! thanks for your patience with this long note!!!