85 300zx throttle and stalling problem
 

My z sometimes for example when I reverse out of my driveway and stop at the top to close the garage and take off again the engine revs up but the car dosen't go anywhere and then suddenly takes off as though the car is out of gear it happens in either drive or reverse and sometimes the car stalls when I start it first thing of a day when I am out of the garage and stop at the top to close it the car stalls but when the car is warmed up it is fine.
The car is an auto.
Any help would be great.