Trasmission problems

Hello,

I have a 2000 mercury sable ls, since then end of last summer my car has had some trouble shifting after I drive for 30-60 minutes

but only when its warm out, I didnt have any trouble all winter long but last week when the temperature got up around 80

it started doing it again.

It will run fine for awhile but then after 30-60m it takes a little while to shift

i had the fluid checked and it was full and not burnt any help would be appreciated

thanks

jared