at certain times..

At
one point I will be at a place where I stop the car, turn it off and walk
....(when I turn off the car and the engine cooling fan is
ON)...when I start the car again after about 5 seconds, it will chug and
bounce and its idling very rough..(.the oil gauge will go up and down a bit
too.)..start driving and within a few seconds, it evens out and its fine.
IF I turn off the car and the engine cooling fan is NOT on, then it doesn't do the
chugging bounce thing. Its been doing this for about 2 months now.

replaced the air flow sensor..that didn't do it... any ideas? Is it the oxygen sensor or something else. It only happens when I turn the car off while the cooling fan is ON..if its off, it doesnt do it.