help please

Hi, I have a 1996 Astro passenger van with automatic transmission, that works really well until I travel a far distance on highway of 20 miles or so, the transmission will shift very hard on the first gear only, I notice the hard shifting when I stop to paid the toll and on every traffic light. But if I used the van all day driving local, the transmission will shift all gear really nice and smooth it work really well but once I get on the highway and travel 20 miles or more, the transmission will start shifting very hard on fist gear and smooth on the other, I will really appreciated any help I can get.

Thank you