Transmission is slipping

I have a 1993 Dodge Caravan mileage is 160,000 miles engine size is 3.5 L V6
I can drive my van for about an hour and when the transmission heats up it starts slipping a little bit. When it started slipping, the tranny fluid and filter was changed, and it slipped a little afterwards. When it starts slipping all I gotta do is pull over and let it cool down and it drives fine. I am really hoping this doesnt mean that my transmission is going out, I would welcome any comments, suggestions or ideas as to what is going on. Thanks much