1990 Nissan 240sx automatic 4 cyl. 175,000 miles

I had a shop change the transmission in my 1990 240sx with one from a 1996. It shifts fine until it goes up a steep grade and then it seems to go out of gear until it slows down a little. Ive had two shops try to figure this out . Is this a computer problem and is a 1996 transmission compatible with a 1990? Thank's Bear