try servicing the transmission
check the fluid, if its dark then it probably does need servicing.

when you do so remove the sump screen and clean it out
check it for the amount of debris in the screen while your at it.
a partially clogged sump screen or low fluid level will cause this problem.


if you do not find any problems there then remove the shifting solenoid from the top of the transmission and clean it, it may be sticking. If cleaning it improves the operation then order a new valve, generally even cleaning them will not fix it due to the internal comtamination, i've never been able to clean one good enough for it to last very long afterwards so i always wound up replacing it in the end.

you may have to do both cleaning the sump screen and the valve, it all depends on how dirty the fluid is.