What is it on 2003 Pontiac Bonneville

I have a 2003 Pontiac bonneville that started out shifting into 2&3 gear hard and over reving so I got some Lucus trans fluid that stoped slips and the next day I pulled up to a stop sighn and when I pushed the gas to go car wouldn't move in any gear. So I shut the car off and re started it and put it in drive and it worked fine but it keeps doing it now what do you think it is?

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1 answer 2 comments
Trans is shot.
But why would it move and work good after I shut the car down then it would do it again
Internal leaks that get worse as it warms up and expansion occurs. When is the last time the pans been off and filter serviced?