New a new transmission ?

I've had this problem with my Kia spectra for a while now. The car has an automatic transmission it's gotten to the point where I always have to shift to neutral when I'm slowing down and put it back into drive when I want to move forward. If I don't do this and try to accelerate it won't go into first gear, the engine will just rev up and the car won't move. Usually have to turn off and turn it back on in the middle of the street to get going again. If you accelerate took quickly from rest the same thing will happen. If you break hard and try to accelerate it will also slip and won't go anywhere, kinda scary if ur on the highway, which has happened to me already. Once you get a steady speed it shifts into gears just fine. The fluid level is just fine and it's clean. Could also be a torque converter problem ?