What would cause my Transmission to when I an going up a slight incline to shift on 2001 Hyundai Elantra

for it to shift down and mand want change back I have to stop and turn Key off and it will click like everything shuts down and then I can start it back and it is fine

Asked by for the 2001 Hyundai Elantra
seek a free diag and est from aamco then shop prices