1992 pontaic grand am 157965miles

I have a 1992 pontaic grand am with 157965 miles on the car..I love this car it has been a very good car but i am so fustrated because I have had the same problem with the car for over 6 months now and can't figure out what is wrong..First of all it does this ever so often..and then for a whole week my car drives fine with no problems..when i start the car up the engine idles really fast. And when the engine slows down it starts to jerk back and fourth. and then when i take it out of park to either put in in reverse or drive and take the brake pedal off, The car just goes with out putting the gas on..When i do put the gas on the speedometer goes from 0 to 120 in less then a second. and then it jumbs from 90 to 120 and then when I am at a red light it idles fast and then the engine starts to jerk again...Sometimes it jerks when i let go of the gas petal... Sometimes the car will fix it self and go back to normal like the speedometer will go back to what speed i really am going and then the car doesn't jump or idle really fast when is start it up again. But then A week later I will have the same problems with it. I have changed everything i can think of...the map sencor,oxygen sencor,IAC sencor, speed sencor. I don't know what else to do..I love my car and would like to have it abit longer if possible PLEASE HELP ME!!!!!!