some wrong in my car

well, i have a 95 eclipse gs, before 2 month ma car would turn on but work it get warmed up the idle start going up and down so fast and then it start to turn off and smoke start coming off. so i took it a mechanic and thought it was a computer that is messed but at the end it was a censor under the car below the engine and after that the car ran good. but now my car have 189,940 miles on it and when u drive its perfect but when u stop the idle goes down a little bit to less than 1000 rpm and the car start shakin and smoke start coming out white smoke does any body know wut is wrong with it please tell me thx