1998 Pontiac Bonneville transmission performance

I have a 1998 bonnieville non turbo. when you drive in town and go to turn at intersection the car acts like it doesnt shift all the way down into first. and the shifts hard. it does not do it all the time just on occasion and is annoying. if you pull over and turn car off it seems to fix the problem and it does fine then. it does not do it everyday. I changed the trans filter and fluid. the old fluid was dark and smelt burnt. after cahmging fluid the car ran and drove like a dream for three days and not has started doing it again. what is wrong with it. mike