Nissan micra 1998 R 1.3 automatic very knotchy gear selection?

I have a 1998 R 1.3 Nissam Micra automatic, that is very knotchy (stiff) thru drive_ nutural _ Reverse.
I have topped up the trnsmission fluid, & tried getting the engine hot before attempting to select a gear, but it makes no difference, is it clutch or agearbox problem ? & what can i do to cure it ?
thanks Bob